Consider drain iterator for `Copy`/`Clone` types

[sgodwincs]

We currently cannot implement a generic insertion ordered drain iterator for ListOrderedMultimap due to the constraint that keys must remain in the map until all values have been drained. The pop_back and pop_front are workarounds for this limitation by providing KeyWrappers instead of moving the key out of the map.

A similar KeyWrapper approach cannot be done with an actual iterator due to lifetime limitations (i.e. it's not safe to call next while the previous items yielded have yet to have been dropped, this cannot be enforced through the iterator trait). But if we add the constraint that the Key type is Copy or potentially Clone, we can add a drain iterator and just copy/clone the keys when iterating. Doing it with Clone seems like a bit too much as it can potentially have a massive performance impact, but perhaps this should just be the user's responsibility.

[sgodwincs]

Also, the exact same approach can be done for IntoIterator. Providing this and the drain iterator will allow the multimap to be used in a greater number of situations, especially with functions taking collections which must be able to be converted into an iterator.