Improve indexing performance benchmarks

[crusaderky]

As discussed in #3375 - FYI @jhamman

asv_bench/benchmarks/indexing.py is currently missing some key use cases:

• All tests in the above module use arrays with 2~6 million points.
  While this is important to spot any case where the numpy underlying functions start being unnecessarily called more than once, it also means any performance improvement or degradation in any of the pure-Python code will be completely drowned out.
  All tests should be run twice, once with the current nx = 3000; ny = 2000; nt = 1000 and again with nx = 15; ny = 10; nt = 5.
• DataArray slicing (sel, isel, and square brackets)
• Slicing when there are no IndexVariables (verify that we're not creating dummy variables, doing a full scan on them, and then discarding them)
• other?