[hashmap] Adds `drain`: a way to sneak out the elements while clearing a map.

It is useful to move all the elements out of a hashmap without deallocating
the underlying buffer. It came up in IRC, and this patch implements it as
`drain`.

r? @Gankro
cc: @frankmcsherry
Word on high from lord @aturon is that these should be called Drain (as Iterators should be named after their creation method -- Iter, IterMut, IntoIter.