trunnel: Add subproto request definition

This essentially implements prop346 (ntorv3 subprotocol) and prop340 (packed and frag).

In this branch, the CONFLUX_SWITCH cell is packed. Not the XON/XOFF just yet. The complexity of that MR is already very high and so I want to let the reviewer consider how we do it for the SWITCH before going on for the others.

Signed-off-by: David Goulet dgoulet@torproject.org