Add steal_many

[ghost]

This PR merges fifo.rs and lifo.rs together, adds new method steal_many, and changes the return type of pop.

Benchmarks in tokio-threadpool:

 name                    before ns/iter  after ns/iter  diff ns/iter  diff %  speedup
 threadpool::spawn_many  3,563,790       3,548,500           -15,290  -0.43%   x 1.00
 threadpool::yield_many  12,171,989      11,606,183         -565,806  -4.65%   x 1.05

Some of the wins are due to better optimized spinning (it's handled in tokio with Steal::Retry, while previously spinning was entirely contained inside Stealer::steal).

Closes #4

[ghost]

Rayon benchmarks:

 factorial::factorial_iterator                                       19,197,280             19,288,249                   90,969    0.47%   x 1.00
 factorial::factorial_join                                           2,674,038              2,856,833                   182,795    6.84%   x 0.94
 factorial::factorial_par_iter                                       2,557,865              2,674,078                   116,213    4.54%   x 0.96
 factorial::factorial_recursion                                      3,082,174              3,176,094                    93,920    3.05%   x 0.97
 fibonacci::fibonacci_iterative                                      6                      6                                 0    0.00%   x 1.00
 fibonacci::fibonacci_join_1_2                                       57,271,590             49,210,576               -8,061,014  -14.08%   x 1.16
 fibonacci::fibonacci_join_2_1                                       48,046,315             48,232,800                  186,485    0.39%   x 1.00
 fibonacci::fibonacci_recursive                                      11,010,701             11,839,683                  828,982    7.53%   x 0.93
 fibonacci::fibonacci_split_iterative                                27,170                 30,079                        2,909   10.71%   x 0.90
 fibonacci::fibonacci_split_recursive                                6,336,907              6,431,466                    94,559    1.49%   x 0.99
 find::size1::parallel_find_common                                   7,215                  7,296                            81    1.12%   x 0.99
 find::size1::parallel_find_first                                    4,220                  4,852                           632   14.98%   x 0.87
 find::size1::parallel_find_last                                     3,811,913              3,959,520                   147,607    3.87%   x 0.96
 find::size1::parallel_find_middle                                   2,545,866              2,526,675                   -19,191   -0.75%   x 1.01
 find::size1::parallel_find_missing                                  3,938,847              4,034,027                    95,180    2.42%   x 0.98
 find::size1::serial_find_common                                     1,939                  1,936                            -3   -0.15%   x 1.00
 find::size1::serial_find_first                                      1                      1                                 0    0.00%   x 1.00
 find::size1::serial_find_last                                       4,138,680              4,165,122                    26,442    0.64%   x 0.99
 find::size1::serial_find_middle                                     2,860,534              2,780,353                   -80,181   -2.80%   x 1.03
 find::size1::serial_find_missing                                    4,130,625              4,157,499                    26,874    0.65%   x 0.99
 join_microbench::increment_all                                      38,484                 43,734                        5,250   13.64%   x 0.88
 join_microbench::increment_all_atomized                             2,564,093              2,501,344                   -62,749   -2.45%   x 1.03
 join_microbench::increment_all_max                                  84,965                 89,989                        5,024    5.91%   x 0.94
 join_microbench::increment_all_min                                  29,631                 30,564                          933    3.15%   x 0.97
 join_microbench::increment_all_serialized                           39,819                 39,958                          139    0.35%   x 1.00
 join_microbench::join_recursively                                   933,041                886,676                     -46,365   -4.97%   x 1.05
 life::bench::as_parallel_generations                                522,270,832            522,470,226                 199,394    0.04%   x 1.00
 life::bench::generations                                            130,343,907            129,888,951                -454,956   -0.35%   x 1.00
 life::bench::parallel_generations                                   55,533,131             56,858,894                1,325,763    2.39%   x 0.98
 map_collect::i_mod_10_to_i::with_collect                            9,001,844              9,132,965                   131,121    1.46%   x 0.99
 map_collect::i_mod_10_to_i::with_fold                               3,984,070              4,203,013                   218,943    5.50%   x 0.95
 map_collect::i_mod_10_to_i::with_fold_vec                           4,538,755              4,691,997                   153,242    3.38%   x 0.97
 map_collect::i_mod_10_to_i::with_linked_list_collect                17,011,076             16,584,602                 -426,474   -2.51%   x 1.03
 map_collect::i_mod_10_to_i::with_linked_list_collect_vec            7,716,738              7,834,976                   118,238    1.53%   x 0.98
 map_collect::i_mod_10_to_i::with_linked_list_collect_vec_sized      9,358,593              9,525,245                   166,652    1.78%   x 0.98
 map_collect::i_mod_10_to_i::with_linked_list_map_reduce_vec_sized   9,022,822              9,034,552                    11,730    0.13%   x 1.00
 map_collect::i_mod_10_to_i::with_mutex                              69,347,858             72,063,225                2,715,367    3.92%   x 0.96
 map_collect::i_mod_10_to_i::with_mutex_vec                          10,452,009             10,824,518                  372,509    3.56%   x 0.97
 map_collect::i_mod_10_to_i::with_vec_vec_sized                      8,910,886              9,023,208                   112,322    1.26%   x 0.99
 map_collect::i_to_i::with_collect                                   28,606,999             28,347,388                 -259,611   -0.91%   x 1.01
 map_collect::i_to_i::with_fold                                      77,027,845             82,575,900                5,548,055    7.20%   x 0.93
 map_collect::i_to_i::with_fold_vec                                  76,499,403             82,074,206                5,574,803    7.29%   x 0.93
 map_collect::i_to_i::with_linked_list_collect                       38,915,261             38,009,858                 -905,403   -2.33%   x 1.02
 map_collect::i_to_i::with_linked_list_collect_vec                   37,905,973             38,577,767                  671,794    1.77%   x 0.98
 map_collect::i_to_i::with_linked_list_collect_vec_sized             30,016,425             29,340,421                 -676,004   -2.25%   x 1.02
 map_collect::i_to_i::with_linked_list_map_reduce_vec_sized          28,537,139             28,661,547                  124,408    0.44%   x 1.00
 map_collect::i_to_i::with_mutex                                     128,029,957            130,934,487               2,904,530    2.27%   x 0.98
 map_collect::i_to_i::with_mutex_vec                                 50,142,918             51,628,736                1,485,818    2.96%   x 0.97
 map_collect::i_to_i::with_vec_vec_sized                             28,930,722             28,621,925                 -308,797   -1.07%   x 1.01
 matmul::bench::bench_matmul_strassen                                6,531,975              6,559,291                    27,316    0.42%   x 1.00
 mergesort::bench::merge_sort_par_bench                              9,409,559              9,289,732                  -119,827   -1.27%   x 1.01
 mergesort::bench::merge_sort_seq_bench                              27,773,968             27,679,008                  -94,960   -0.34%   x 1.00
 nbody::bench::nbody_par                                             18,855,225             18,809,061                  -46,164   -0.24%   x 1.00
 nbody::bench::nbody_par_bridge                                      18,339,773             19,174,396                  834,623    4.55%   x 0.96
 nbody::bench::nbody_parreduce                                       38,060,868             39,151,065                1,090,197    2.86%   x 0.97
 nbody::bench::nbody_seq                                             42,088,394             42,535,883                  447,489    1.06%   x 0.99
 pythagoras::euclid_faux_serial                                      33,562,839             33,682,226                  119,387    0.36%   x 1.00
 pythagoras::euclid_parallel_full                                    73,998,851             73,639,899                 -358,952   -0.49%   x 1.00
 pythagoras::euclid_parallel_one                                     12,173,047             12,032,539                 -140,508   -1.15%   x 1.01
 pythagoras::euclid_parallel_outer                                   12,095,393             11,898,078                 -197,315   -1.63%   x 1.02
 pythagoras::euclid_parallel_weightless                              12,085,512             12,101,750                   16,238    0.13%   x 1.00
 pythagoras::euclid_serial                                           33,247,090             30,084,810               -3,162,280   -9.51%   x 1.11
 quicksort::bench::quick_sort_par_bench                              16,571,532             16,411,665                 -159,867   -0.96%   x 1.01
 quicksort::bench::quick_sort_seq_bench                              40,344,696             40,080,376                 -264,320   -0.66%   x 1.01
 quicksort::bench::quick_sort_splitter                               21,040,440             19,649,396               -1,391,044   -6.61%   x 1.07
 sieve::bench::sieve_chunks                                          10,510,473             10,438,599                  -71,874   -0.68%   x 1.01
 sieve::bench::sieve_parallel                                        6,512,114              6,519,326                     7,212    0.11%   x 1.00
 sieve::bench::sieve_serial                                          23,788,687             24,117,128                  328,441    1.38%   x 0.99
 sort::demo_merge_sort_ascending                                     132,291 (3023 MB/s)    131,572 (3040 MB/s)            -719   -0.54%   x 1.01
 sort::demo_merge_sort_big                                           10,342,530 (618 MB/s)  10,402,822 (615 MB/s)        60,292    0.58%   x 0.99
 sort::demo_merge_sort_descending                                    166,150 (2407 MB/s)    167,263 (2391 MB/s)           1,113    0.67%   x 0.99
 sort::demo_merge_sort_mostly_ascending                              365,265 (1095 MB/s)    363,908 (1099 MB/s)          -1,357   -0.37%   x 1.00
 sort::demo_merge_sort_mostly_descending                             409,707 (976 MB/s)     405,144 (987 MB/s)           -4,563   -1.11%   x 1.01
 sort::demo_merge_sort_random                                        1,716,228 (233 MB/s)   1,698,890 (235 MB/s)        -17,338   -1.01%   x 1.01
 sort::demo_merge_sort_strings                                       5,012,530 (159 MB/s)   5,115,864 (156 MB/s)        103,334    2.06%   x 0.98
 sort::demo_quick_sort_big                                           8,165,314 (783 MB/s)   8,246,772 (776 MB/s)         81,458    1.00%   x 0.99
 sort::demo_quick_sort_mostly_ascending                              18,654,121 (21 MB/s)   19,211,662 (20 MB/s)        557,541    2.99%   x 0.97
 sort::demo_quick_sort_mostly_descending                             11,903,002 (33 MB/s)   12,383,304 (32 MB/s)        480,302    4.04%   x 0.96
 sort::demo_quick_sort_random                                        1,478,722 (270 MB/s)   1,476,785 (270 MB/s)         -1,937   -0.13%   x 1.00
 sort::demo_quick_sort_strings                                       6,241,767 (128 MB/s)   6,225,578 (128 MB/s)        -16,189   -0.26%   x 1.00
 sort::par_sort_ascending                                            61,786 (6473 MB/s)     63,897 (6260 MB/s)            2,111    3.42%   x 0.97
 sort::par_sort_big                                                  11,183,151 (572 MB/s)  11,191,287 (571 MB/s)         8,136    0.07%   x 1.00
 sort::par_sort_descending                                           117,688 (3398 MB/s)    122,058 (3277 MB/s)           4,370    3.71%   x 0.96
 sort::par_sort_expensive                                            68,791,700 (5 MB/s)    69,350,609 (5 MB/s)         558,909    0.81%   x 0.99
 sort::par_sort_mostly_ascending                                     377,197 (1060 MB/s)    383,283 (1043 MB/s)           6,086    1.61%   x 0.98
 sort::par_sort_mostly_descending                                    409,727 (976 MB/s)     415,129 (963 MB/s)            5,402    1.32%   x 0.99
 sort::par_sort_random                                               1,326,204 (301 MB/s)   1,337,224 (299 MB/s)         11,020    0.83%   x 0.99
 sort::par_sort_strings                                              4,226,276 (189 MB/s)   4,214,945 (189 MB/s)        -11,331   -0.27%   x 1.00
 sort::par_sort_unstable_ascending                                   48,396 (8265 MB/s)     48,464 (8253 MB/s)               68    0.14%   x 1.00
 sort::par_sort_unstable_big                                         6,816,799 (938 MB/s)   6,801,822 (940 MB/s)        -14,977   -0.22%   x 1.00
 sort::par_sort_unstable_descending                                  71,187 (5619 MB/s)     71,601 (5586 MB/s)              414    0.58%   x 0.99
 sort::par_sort_unstable_expensive                                   79,047,756 (5 MB/s)    77,118,163 (5 MB/s)      -1,929,593   -2.44%   x 1.03
 sort::par_sort_unstable_mostly_ascending                            321,047 (1245 MB/s)    325,111 (1230 MB/s)           4,064    1.27%   x 0.99
 sort::par_sort_unstable_mostly_descending                           342,379 (1168 MB/s)    352,487 (1134 MB/s)          10,108    2.95%   x 0.97
 sort::par_sort_unstable_random                                      742,242 (538 MB/s)     747,151 (535 MB/s)            4,909    0.66%   x 0.99
 sort::par_sort_unstable_strings                                     4,237,318 (188 MB/s)   4,285,355 (186 MB/s)         48,037    1.13%   x 0.99
 str_split::parallel_space_char                                      1,337,651              1,343,127                     5,476    0.41%   x 1.00
 str_split::parallel_space_fn                                        903,802                904,327                         525    0.06%   x 1.00
 str_split::serial_space_char                                        2,993,653              3,073,927                    80,274    2.68%   x 0.97
 str_split::serial_space_fn                                          1,635,689              1,529,288                  -106,401   -6.50%   x 1.07
 str_split::serial_space_str                                         2,305,368              2,300,641                    -4,727   -0.21%   x 1.00
 tsp::bench::dj10                                                    11,421,117             11,189,734                 -231,383   -2.03%   x 1.02
 vec_collect::vec_i::with_collect                                    3,809,172              3,993,316                   184,144    4.83%   x 0.95
 vec_collect::vec_i::with_collect_into_vec                           3,794,252              3,994,567                   200,315    5.28%   x 0.95
 vec_collect::vec_i::with_collect_into_vec_reused                    1,817,590              2,330,346                   512,756   28.21%   x 0.78
 vec_collect::vec_i::with_fold                                       50,996,670             54,280,940                3,284,270    6.44%   x 0.94
 vec_collect::vec_i::with_linked_list_collect_vec                    41,426,209             40,348,919               -1,077,290   -2.60%   x 1.03
 vec_collect::vec_i::with_linked_list_collect_vec_sized              34,966,234             35,896,233                  929,999    2.66%   x 0.97
 vec_collect::vec_i::with_linked_list_map_reduce_vec_sized           24,610,179             24,858,848                  248,669    1.01%   x 0.99
 vec_collect::vec_i::with_vec_vec_sized                              24,259,872             25,105,698                  845,826    3.49%   x 0.97
 vec_collect::vec_i_filtered::with_collect                           31,520,106             25,033,713               -6,486,393  -20.58%   x 1.26
 vec_collect::vec_i_filtered::with_fold                              54,750,519             54,326,315                 -424,204   -0.77%   x 1.01
 vec_collect::vec_i_filtered::with_linked_list_collect_vec           45,404,376             41,431,847               -3,972,529   -8.75%   x 1.10
 vec_collect::vec_i_filtered::with_linked_list_collect_vec_sized     39,431,510             36,750,466               -2,681,044   -6.80%   x 1.07
 vec_collect::vec_i_filtered::with_linked_list_map_reduce_vec_sized  31,512,823             24,818,102               -6,694,721  -21.24%   x 1.27
 vec_collect::vec_i_filtered::with_vec_vec_sized                     31,636,532             25,005,764               -6,630,768  -20.96%   x 1.27

[ghost]

I'm going to merge this now so that tokio can start using it. If there are any concerns with this PR, feel free to comment here.