SIMD Support

[rust-highfive]

RFC for improving SIMD support.

First time I've done something like this, so apoligies for anything I may have gotten wrong.

Acknowledgements to @jensnockert's post here: http://blog.aventine.se/2013/07/16/my-vision-for-rust-simd.html for forming the basis of this RFC

Tracking issue for postponed PR #15

[nrc]

See https://github.com/rust-lang/rfcs/blob/master/text/1199-simd-infrastructure.md

[petrochenkov]

Closing in favor of #2325