It's already Sync because it's safe to access amongst many threads by the sheer
fact that data is only accessed on one thread. That is, all other concurrent
accessors, if any, will receive `None`.
This type acts for a handle to storage of non-`Send` data. The handle itself is
sendable across threads and is therefore suitable for storage in a `Future`.
This data uses communication internally and a new method on `Task` to ensure
that when the data needs to be accessed the future will find its way to the
right thread.
More on this type coming soon!
Alex Crichton