Oibit send and friends

More work on opt-in built in traits. `Send` and `Sync` are not opt-in, `OwnedPtr` renamed to `UniquePtr` and the `Send` and `Sync` traits are now unsafe.

NOTE: This likely needs to be rebased on top of the yet-to-land snapshot.

r? @nikomatsakis 
We may want to avoid constructors or functions on this pointer type for now in terms of stabilization. Some questions that arise:

* Why have a `null` constructor while not having constructors for other types of pointers?
* Why have an `offset` function without the other array of functions on raw pointers?
* Should this implement the raw pointer traits?