256- and 512-bit vector types

[emberian]

I don't know whether LLVM lowers these intelligently when not supported, but real hardware (AVX/AVX2 and AVX-512F) support these register sizes, for a nice speed boost for code which can use it. It'd be nice to have types like u32x8 and u32x16 exposed.

[BurntSushi]

@cmr The 256 bit vectors are defined here: https://github.com/rust-lang-nursery/simd/blob/master/src/v256.rs#L45-L46 --- But it looks like they aren't exposed?