SIMD is stable

[coder543]

Now that SIMD has reached stable Rust, it would be great if the SIMD acceleration could be moved out of the unstable feature!

[gnzlbg]

I can remove the dependency on the nightly stdsimd feature and I could easily remove the dependency on align_offset by reimplementing it, but I can't remove the dependencies on specialization and fn_traits :/

I'll leave this open to remove the stdsimd feature, but until specialization and fn_traits become stable unstable will still be required :/

If anybody has any ideas about how to remove the dependency on specialization, PRs welcome.