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 
With [RFC 356](https://github.com/rust-lang/rfcs/blob/master/text/0356-no-module-prefixes.md), this type may wish to be called `Unique` instead of `UniquePtr`.