SIMD for Rust, take 2

[sanxiyn]

At the moment this only includes type checking and there is no code generation support yet. I wanted to get the design reviewed first.

From discussion with @graydon at #5841, re-implemented as #[simd] attribute on structs.

Progressing towards #3499.

[sanxiyn]

One issue I found is that it is no longer possible to decide whether type is a SIMD vector solely from type. You also need type context. This means, for example, pub fn type_is_immediate(ty: t) -> bool signature needs to change.

Or do you have a better idea?

[graydon]

This looks pretty good to me. Thanks for the patient revisions.

Since I am less aware of what servo will need / want in this area (and haven't written SIMD code in years) perhaps @pcwalton should also take a look to confirm?

[sanxiyn]

Rebased.

[brson]

Can we put the library parts in core::unstable? This is an experimental feature.

[yichoi]

sounds reasonable to locate core::unstable

[sanxiyn]

Moved.

[nikomatsakis]

@sanxiyn I read over the patch. It looks pretty good to me so far.