prop224: ntor handshake implementation
Ticket created after https://trac.torproject.org/projects/tor/ticket/20657#comment:12
Initial reviews are here: https://gitlab.com/asn/tor/merge_requests/13
OK after a review from David and some comments from Nick I present the
prop224-ntor-v2 branch which comes with all the code review fixes, and with a full on integration test suite similar to the
./src/test/test_ntor.sh tests for simple ntor.
It also implements the key expansion functionality as requested by David.