core::rt: Add RemoteCallback trait and uv implementation

This is used for signalling the event loop from other threads.
core::rt: Add SchedHandle type
core::rt: Scheduler takes a WorkQueue

This will be for implementing a work-sharing strategy
core::rt: Remove UvEventLoop::new_scheduler function
core::rt: Rename WorkQueue methods, remove extra push method
Implement work-stealing deque for task queue
More workqueue changes
Add another concurrent test