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 
I figured we would land `UniquePtr` can call it experimental or unstable for now until we had a chance to debate it. It's not user-visible in any case.