feat: add drain method

[willothy]

WIP

Adds a method to drain the map with an iterator.

Needs more testing (what happens with deletions while iterating, etc.)

I also think the resize logic is probably wrong because it could maybe lead to elements being seen twice

[ibraheemdev]

Hmm this seems tricky to implement concurrently. I think you would have to block for individual elements to complete to avoid yielding them twice. What's your use case?

[willothy]

Yeah, definitely tricky. Still thinking it through, hence the draft status. Use case is removing and collecting all keys into a Vec. Currently just using .iter().collect() and then .clear().