implement Entry API for SmallIntMap

The contents of this PR make me legit mad. I really hope that I'm just completely stupid tonight, and there's a way better way to do this. It doesn't help that this API is *basically* pointless to provide on SmallIntMap. But we've gotta provide a uniform interface, so...

yeah 

:rage:
Another alternative would be storing `&'a mut Option<V>`, which would be totally safe but would require unwrapping to `Option`.