From/Into implementations between vectors and tuples/arrays

[gnzlbg]

@Centril mentioned it here: rust-lang/rfcs#2366 (comment)

For arrays it should be trivial to implement these using the {load,store}_unaligned API. For tuples, it depends on how the tuple layout is specified, if at all.

Generating these from the macros should be pretty straight forward to do for array. For tuples it might be less straight forward but doable.

[gnzlbg]

This has been fixed in: https://github.com/gnzlbg/packed_simd which incorporates From impls from arrays.