Implement HashMap.find_with_or_insert_with()

This used to be called `mangle` and 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.

The method is renamed to `find_with_or_insert_with`, also with the parameters swapped to make sense—find and then insert, not insert and then find.

/cc @cgaebel