Implement counter galois onion relay encryption in Arti (P112 O3.2)

This is a ticket for a complete implementation of proposal 359, possibly amended as warranted.

To deliver this correctly, we'll need more from proposal 346 (for protover negotiation), and proposal 358 (for unified circuit extension namespaces). We'll also need the small part of proposal 340 (the new relay cell format) that implements the new RELAY cell format.

Blocked by:

• #1943 (closed) - CGO crypto implementation
• #1944 (closed) - relay cell format
• #1945 (closed) - subprotocol-based negotiation (prop346)
• #1946 (closed) - circuit extension unification (prop358)
• #1947 (closed) - negotiate CGO with relays.

Related to:

• #1948 - negotiate CGO with HS. (Not for Project 112)
• tor#41047 (closed) - CGO in C Tor