feature: Add drain_key_if and remove_key_if

[shanecelis]

Thank you for the excellent crate. It's a perfect fit for what I wanted.

I added two functions drain_key_if and remove_key_if that I needed.

In making those work I also added an ExhaustIter behind the scenes that consumes an iterator even when it's dropped prematurely. Perhaps that's controversial. If you don't want that, let me know and I'll take it out.

Thank you for the crate!

[imartayan]

Hello @shanecelis, thank you for your contribution!
I think these functions are useful and having an exhaust iterator for drain_key_if is totally reasonable.
I will also update drain_key to do the same, it seems like a better default.