collections: Reorder slice methods to improve API docs

We have an evolutionary history whose traces are still visible in the
slice docs today.

Some heuristics:

* Group method and method_mut together
* Group method and method_by together
* Group by use case, here we have roughly:

  Basic interrogators (len)
  Mutation (swap)
  Iterators (iter)
  Segmentation (split)
  Searching (contains)
  Permutations (permutations)
  Misc (clone_from_slice)