[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
This also creates a pointer that's out of bounds of allocated memory. That sounds really sketchy to me.

But alas, that's how vec iterators work today.