A bare-pointer implementation for refcount::Ref

With this, we need just the raw function table from Refcount,
no need for the default-impl methods. Also, the generic impl
of Ref got slim.
Added SyncRef and a private trait Threadsafe

This reference type is for sendable, synchronized wrappers
of GLib objects such as GMainLoop and GMainContext.

There is a need to pass these objects into sendable closures, for example
to quit the mainloop, or to send callback closures to another task's
mainloop.