libextra: Introduce typed arenas.

A typed arena is a type of arena that can only allocate objects of one
type. It is 3x faster than the existing arena and 13x faster than malloc
on Mac.
auto merge of #11358 : pcwalton/rust/typed-arenas, r=alexcrichton

A typed arena is a type of arena that can only allocate objects of one
type. It is 3x faster than the existing arena and 13x faster than malloc
on Mac.

r? @brson