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 

cc #13231 
Putting the pointer behind `Unique` makes it `Send` and `Sync`. If we don't do it, we won't be able to *move* it into the closure.