Switch from adler2 to simd-adler32 crate when using miniz_oxide backend

[Shnatsel]

Switch over to https://crates.io/crates/simd-adler32 for a performance boost.

Introduces some unsafe code into the dependency tree due to the use of raw CPU intrinsics. However, this unsafe code is low risk due to the way it's structured, and the crate has also undergone extensive fuzzing.

This has been enabled in the famously risk-averse image-png for a good while now, and is even enabled in the Chromium field trials on stable channel.

It's worth noting that Rust v1.87 has dramatically regressed performance of lots of SIMD code relying on runtime feature etection; this negatively affected the AVX2 codepath of simd-adler32, see rust-lang/rust#142603
Which is why I didn't open this earlier.

On all other compiler versions dating back all the way to MSRV this crate is dramatically faster than adler2.

[Shnatsel]

CI fails because Clippy complains about code I didn't touch.

[Byron]

Thanks so much! Particularly the motivation and explanation was helpful as otherwise I would have worried that adding SIMD instructions will reduce compatibility and break downstream.

Also, I have fixed the clippy warnings, including additional ones that come up with cargo clippy --all-targets.