Tracking issue for `std::simd`

[carols10cents]

Rust tracking issue: rust-lang/rust#27731

Place that needs to be updated: https://github.com/rust-lang-nursery/edition-guide/blame/68c458b9b68edced39aebea60e672a22d4d50edf/src/rust-2018/simd-for-faster-computing.md#L78-L81

We hope to eventually stabilize a std::simd module with higher-level stuff in the future. But landing the basics now lets the ecosystem experiment with higher level libraries starting today.

[ehuss]

Closing per #232, this section has been removed.