Instead of transforming between return value and callback, pass
the wrapped callback on to the invoked function as the last argument.
This eliminates the need for multiple invokeWithArgument overloads
and permits invoked functions to be asynchronous.
Implements a Thread<>::invoke interface that only permits calling
member functions with bound arguments.
Also introduce invokeWithResult, which accepts a callback and invokes
it, passing the result, on the invoking thread's RunLoop. This closes
the loop of two evented threads doing message passing.
