Add unordered element iterator or visitor (using closure)

[bluss]

Like .iter(), but visits the array elements in "arbitrary" order. Will use memory order or whatever order is most efficient.

[bluss]

I'll add that unfortunately by how llvm optimizes, it doesn't seem to be possible to do our very best for segmented arrays here (where one dimension is contiguous, but the whole array is not).

If we write what amounts to a "nested" loop using usual rust external iterators, we need to rely on llvm to recognize it as a nested loop, but this seems to never happen. As you can see, current zip_with_mut uses explicit nested for loops, and at the moment, this does not seem possible to replicate with iterators in any way. Also discussed for the simpler case of a deque in issue rust-lang/rust/issues/30805