implement Entry API for SmallIntMap
auto merge of #17879 : Gankro/rust/smallint-entry, r=aturon

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: