Made Forwarders optionally use transformations.
Simplified the semantics of `Forwarder`.

Also finished renaming `Forwarder.forward()` -> `Forwarder.send()` to
prepare for the use of `TwoWayForwarder` objects.
Better buffering of user data going forward.

The `OnionCircuit.forward` forwarder object has been setup to buffer
user data until a deferred from the `OnionCircuit._handshake` object
(indicating that the handshake is complete) is fired.