Add SimdArray trait and safe gather/scatter API (rust-lang/stdsimd#139)
This PR has four parts, without which it doesn't make a lot of sense:
- The introduction of the SimdArray trait for abstraction over vectors.
- The implementation of private vector-of-pointers types.
- Using these to allow constructing vectors with SimdArray::gather_{or, or_default, select}.
- Using these to allow writing vectors using SimdArray::scatter{,_select}.
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