class Async::Wrapper

Binding to a reactor is purely a performance consideration. Generally, I don't like APIs that exist only due to optimisations. This is borderline, so consider this functionality semi-private.
Bind this wrapper to a different reactor. Assign nil to convert to an unbound wrapper (can be used from any reactor/task but with slightly increased overhead.)

def reactor= reactor
	@monitor.close if @monitor
	
	@reactor = reactor
	@monitor = nil
end