Add LSX compare256 implementation

[Gelbpunkt]

Tests pass, I'd love a benchmark run on this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
fuzz-compress	?
fuzz-decompress	?
test-aarch64-apple-darwin	89.32% <ø> (-0.04%)	⬇️
test-aarch64-unknown-linux-gnu	84.88% <ø> (ø)
test-i686-unknown-linux-gnu	84.35% <ø> (-0.02%)	⬇️
test-x86_64-apple-darwin	88.27% <ø> (-0.03%)	⬇️
test-x86_64-unknown-linux-gnu	89.37% <ø> (-3.68%)	⬇️

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Files with missing lines	Coverage Δ
zlib-rs/src/deflate/compare256.rs	95.90% <ø> (ø)

... and 5 files with indirect coverage changes

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[Gelbpunkt]

I see gains of around 4% on silesia-small at compression level 1, which uses the quick algorithm, that in turn uses compare256:

Benchmark 1 (39 runs): target/release/examples/compress-baseline 1 rs silesia-small.tar
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           129ms ± 1.05ms     128ms …  132ms          0 ( 0%)        0%
  peak_rss           38.2MB ± 17.8MB    25.2MB … 84.2MB          0 ( 0%)        0%
  cpu_cycles          271M  ±  437K      270M  …  272M           0 ( 0%)        0%
  instructions        589M  ±    0       589M  …  589M           0 ( 0%)        0%
  cache_references    336M  ±  180K      335M  …  336M           0 ( 0%)        0%
  cache_misses       28.4M  ±  194K     28.2M  … 28.9M           0 ( 0%)        0%
Benchmark 2 (41 runs): target/release/examples/blogpost-compress 1 rs silesia-small.tar
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           124ms ±  877us     123ms …  127ms          1 ( 2%)        ⚡-  4.0% ±  0.3%
  peak_rss           31.9MB ± 12.3MB    24.4MB … 59.2MB          0 ( 0%)          - 16.4% ± 17.8%
  cpu_cycles          260M  ±  487K      259M  …  261M           0 ( 0%)        ⚡-  4.0% ±  0.1%
  instructions        574M  ±    0       574M  …  574M           0 ( 0%)        ⚡-  2.4% ±  0.0%
  cache_references    315M  ±  185K      314M  …  315M           3 ( 7%)        ⚡-  6.3% ±  0.0%
  cache_misses       26.0M  ±  229K     25.8M  … 26.9M           6 (15%)        ⚡-  8.5% ±  0.3%