use safe chunking in wasm32/aarch64 compare256

[folkertdev]

just copying the changes from the avx2 version

identical assembly is produces

• wasm https://godbolt.org/z/v4c9n9964
• aarch64 https://godbolt.org/z/PY8M3zWo8

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
zlib-rs/src/deflate/compare256.rs	98.06% <100.00%> (ø)

... and 1 file with indirect coverage changes