512-bit types and AVX-512 support

[kytans]

This crate seems to lack 512-bit types and AVX-512 support, which is becoming quite important..

[Lokathor]

The best path to improved 512-bit support is one of two routes:

• land 512-bit intrinsics in stable Rust, then add them to safe_arch, then add to this crate.
• land the portable-simd stuff in stable Rust and just use that with a 512-bit width type.

[Lokathor]

I think most of this has been added in #203