[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
Agreed. This is still useful when I want to move the Iterator around, then recover the collection from the iterator (rather than holding the empty husk of a collection after a .drain).