Functionality for working with SIMD registers and instructions.
`benches/sum-vector.rs`:
```
running 2 tests
test nonsimd ... bench: 2569 ns/iter (+/- 122)
test simd_ ... bench: 611 ns/iter (+/- 23)
test result: ok. 0 passed; 0 failed; 0 ignored; 2 measured
```