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 
This may not necessarily be a public-facing API that we're willing to commit to at this time, could this be moved into just the `std::comm` implementation?