add sse2 variant of compare_256

[folkertdev]

haven't quite decided whether this is worth the lines yet.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
zlib-rs/src/cpu_features.rs	71.42%	2 Missing ⚠️
zlib-rs/src/deflate/compare256.rs	97.36%	1 Missing ⚠️

Files with missing lines	Coverage Δ
zlib-rs/src/deflate/compare256.rs	93.65% <97.36%> (-4.37%)	⬇️
zlib-rs/src/cpu_features.rs	60.93% <71.42%> (+1.28%)	⬆️