packed_simd implementation

[bitshifter]

Long term I think it would make sense to replace the SSE2 implementation with packed_simd. The main reason being that packed_simd will support multiple platforms meaning glam won't need to implement support for other platforms itself.

I always want glam to build on stable so until packed_simd is stabilised, any work on it should live in a branch.

I would probably keep the scalar-math implementation in glam so it can be built without SIMD or custom alignment of types.

There are a few other questions around packed_simd:

• It's in the nursery but development doesn't look very active, is there a plan to get it to stable?
• I what gets generated on platforms that packed_simd doesn't support?
• Is it possible to turn off SIMD

https://github.com/rust-lang-nursery/packed_simd

[bitshifter]

This is on the back burner for now as packed_simd is kind of on hold - rust-lang/packed_simd#213 (comment)

[bitshifter]

Closing until packed simd is looking ready or there's a viable alternative.