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:
What len is being decremented? `SmallIntMap` doesn't cache the number of set elements.