Stable rust

[explosion-mental]

packet_simd only works on stable (and I think portable_simd is it's replacement), so while [that stabilizes](https://github.com/rust-lang/rfcs/pull/2366] I propose having a feature flag for stable rust, either by using another crate or disabling simd all together.

[seijikun]

kmeans was created with SIMD as a design decision to the core, because everything else I tried at the time (including Matlab, a nalgebra implementation, ...) was too slow.
Making it build with stable would mean implementing everything a second time and maintaining it alongside.
Since I'm not using the library myself anymore, I'm afraid that's not in-scope for me. Sorry.
I admit that the naming kmeans might have been rather unfortunate (since too generic).

[explosion-mental]

I understand, thanks for the work being done.

[cyypherus]

If nothing else, it may be useful to document the requirement for unstable rust 🙂