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:
@gereeter Oh! You know, I just totally assumed. That makes the Option solution much more viable, though the unwraps are unsavoury.