A sendable reference type for objects not parked in a thread

[mzabaluev]

There are cases when an object can be sent to another thread as long as no other references to it exist in safe code.
Add a sendable reference type NewRef with the following features:

• Derefs to the object
• Implements Send
• Does not implement Clone
• A method or function, tentatively named park(), promotes a NewRef into Ref, effectively parking the object in the calling thread.

See some rationale in a comment on the GObject-introspection bug regarding thread-safety annotations.