remove explicit SIMD reads and writes

[folkertdev]

Using standard ptr::{read_unaligned, write_unaligned} on an [u8; N] generates the same code. That also means we can just pass the optimal read/write width as a const generic, which is much clearer.

I think we can then also replace the FEATURES const generic with the preferred width, but I'll do that separately later.

[folkertdev]

given our changes to where the target feature is applied, this will not test a simd read/write any more. But those should be semantically equivalent to an array of the corresponding size.

[codecov]

Codecov Report

Attention: Patch coverage is 88.09524% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
zlib-rs/src/inflate/writer.rs	88.09%	5 Missing ⚠️

Files with missing lines	Coverage Δ
zlib-rs/src/inflate/writer.rs	92.02% <88.09%> (+2.85%)	⬆️

... and 2 files with indirect coverage changes