Bump MSRV

[nazar-pc]

CI on master now because later version of rayon requires Rust 1.80.0 and CI is testing against 1.66.1.

It is time to bump MSRV since without lock file it will not really be possible to address this cleanly.

While at it, I'd like to request a newer Rust version so PRs like #439 can finally move forward.

If you bump it all the way to the very latest 1.89.0, then it will be possible to use stable AVX512 intrinsics in Rust without extra ceremony, which would have been nice (I am planning to look into Rust intrinsics closer and make them as fast as assembly to remove the need to deal with C/assembly code).

[nazar-pc]

3a90f0f bumped the minimum to 1.80.1, I guess no luck with newer versions for now 🥲

[oconnor663]

Ah apologies, I didn't see this issue before I did the bump. As far as an MSRV policy, I think something like "stable minus two" is probably reasonable. If you get AVX-512 intrinsics working, I think that's a fine reason to do another bump.