simd_backend: replace packed_simd_2 with std::simd

[tarcieri]

The packed_simd_2 crate used by the simd_backend feature is a prototype of std::simd, which has since landed in the nightly compiler, making packed_simd_2 effectively obsolete.

It would probably be a good idea to migrate simd_backend to use std::simd.

[rozbb]

Looked into this and it seems like a pretty nontrivial change. Some traits aren't implemented (e.g., going from Simd<u32, 8> to a Simd<u64, 4>), shifting like Shl<u8> for Simd<u64, 4>, and extracting lanes Simd<u32, 8>::extract.

Maybe it'd be best to remove the do-for-4.0?

[tarcieri]

Sure

[tarcieri]

I consider this addressed by #520.

An eventual migration to std::simd would probably make sense, but in the meantime, this has removed the pack_simd_2 dependency and better yet made everything work on stable (where std::simd is currently nightly-only).