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 
Yup. Actually, @nikomatsakis had asked me to do that already. I haven't done it yet because I didn't quite understand the need. Now I do, thanks for the comment :D