[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
I know this is stolen from Vec, but I think this is just legacy. As far as I know you can do `as *const _` and ptr::read that ptr.