Feature request: partial drain

[redbaron]

For my usecase I'd like to pop N elements, leaving them vacant. Currently Drain removes all entries. Is it possible to add a variant of drain which on each .next():

• returns by move stored element
• stores Entry::Vacant in place

?

[Bergmann89]

I've recently created a fork of slab, that implements range iterators. With a few adaptions we could implement a draining iterator that accepts a range of items to be drained.