Make hashmap iterators implement ExactSize

I can see the argument either way, honestly. Plus we can explicitly document that behaviour can't be relied on for different sequences of next/next_back. Which is backwards-compatible to "undo". I'm not a huge fan of duplicating a bunch on unsafe code in the already-beefy rawtable. Making next=next_back seems like a simple way to do that.

Interested in getting some other opinions though.