Deque shouldn't require copyable types

[brson]

I want to enqueue non-copyable things!

[graydon]

Dvec and Deque should merge, call it Queue, implement as unsafe if necessary (moving gap through array, should be easy enough to isolate in a couple helpers).

[nikomatsakis]

I believe DVec should be deleted in favor of a type MutVec<T> which is just an alias for Mut<~[T]>.

Perhaps the Queue functionality should just be methods on ~[T]?

[nikomatsakis]

I haven't really explained how the Mut<T> type is supposed to be used though. I've been meaning to try and write something up on how I envision it working.