bitset: add range counting and parallel bit extract/deposit operations

[tsenart]

Add three new operations to BitSet:

• OnesBetween(from, to uint) counts set bits in range [from, to)
• Extract/ExtractTo compacts bits from positions specified by a mask
• Deposit/DepositTo spreads bits to positions specified by a mask

OnesBetween operates at the word level where possible, handling edge words with masks. It returns 0 for invalid ranges where from >= to.

Extract takes bits from positions where mask is set and packs them into consecutive positions starting at 0. For example, if mask has bits set at positions 1,4,5 then Extract will take bits at those positions from the source and pack them into positions 0,1,2.

Deposit is the inverse of Extract - it spreads bits from consecutive positions into positions specified by mask.

The parallel bit operations are implemented using byte-wise lookup tables rather than CPU-specific instructions like PEXT/PDEP, making them portable while maintaining good performance.

Added extensive testing:

• Property-based tests with 1M iterations verifying correctness
• Edge cases like single words, cross-word boundaries, empty sets
• Invariant checking (e.g. result bits ⊆ mask bits)
• Benchmarks across sizes from 64 to 65536 bits

These operations are useful for:

• Efficient bit field compression/decompression
• Sparse set operations
• Bit permutation tasks

The implementation aims to be both correct and fast, operating at the word level where possible while properly handling edges.

Benchmarks

goos: darwin
goarch: arm64
pkg: github.com/bits-and-blooms/bitset
cpu: Apple M3 Max
                                                │    bench    │
                                                │   sec/op    │
BitSetOnesBetween/size=64/density=0.1-16          3.026n ± 1%
BitSetOnesBetween/size=64/density=0.5-16          2.959n ± 3%
BitSetOnesBetween/size=64/density=0.9-16          3.058n ± 1%
BitSetOnesBetween/size=256/density=0.1-16         3.154n ± 2%
BitSetOnesBetween/size=256/density=0.5-16         3.179n ± 1%
BitSetOnesBetween/size=256/density=0.9-16         3.146n ± 1%
BitSetOnesBetween/size=1024/density=0.1-16        4.938n ± 1%
BitSetOnesBetween/size=1024/density=0.5-16        4.934n ± 1%
BitSetOnesBetween/size=1024/density=0.9-16        4.881n ± 1%
BitSetOnesBetween/size=4096/density=0.1-16        8.661n ± 1%
BitSetOnesBetween/size=4096/density=0.5-16        8.787n ± 2%
BitSetOnesBetween/size=4096/density=0.9-16        8.582n ± 1%
BitSetOnesBetween/size=16384/density=0.1-16       29.05n ± 0%
BitSetOnesBetween/size=16384/density=0.5-16       30.11n ± 0%
BitSetOnesBetween/size=16384/density=0.9-16       29.34n ± 0%
PEXT-16                                           3.243n ± 2%
PDEP-16                                           3.219n ± 1%
BitSetExtractDeposit/size=64/fn=ExtractTo-16      10.55n ± 0%
BitSetExtractDeposit/size=64/fn=DepositTo-16      9.812n ± 1%
BitSetExtractDeposit/size=256/fn=ExtractTo-16     35.30n ± 1%
BitSetExtractDeposit/size=256/fn=DepositTo-16     38.16n ± 1%
BitSetExtractDeposit/size=1024/fn=ExtractTo-16    134.5n ± 1%
BitSetExtractDeposit/size=1024/fn=DepositTo-16    157.2n ± 1%
BitSetExtractDeposit/size=4096/fn=ExtractTo-16    580.1n ± 2%
BitSetExtractDeposit/size=4096/fn=DepositTo-16    642.0n ± 1%
BitSetExtractDeposit/size=16384/fn=ExtractTo-16   2.412µ ± 4%
BitSetExtractDeposit/size=16384/fn=DepositTo-16   2.459µ ± 2%
BitSetExtractDeposit/size=65536/fn=ExtractTo-16   10.12µ ± 1%
BitSetExtractDeposit/size=65536/fn=DepositTo-16   9.832µ ± 1%
geomean                                           30.55n