Reinstate HashMap.mangle().

This was removed when the Robin Hood hash map came along, but it is a useful thing to have in certain situations (I just hit it with my Teepee header representation), so I want it back.

I’m not terribly enthusiastic about the name, but there’s historical precedent for `mangle`, so let it be that for now. `find_with_or_insert_with` is what occurs to me as a replacement in keeping with what else is there, but it *is* a bit of a mouthful. I would still be in favour of renaming it to `find_with_or_insert_with` over `mangle`.

/cc @cgaebel