Implement all cell types for onion services

The cells needed for onion services are:

• ESTABLISH_INTRO
• INTRODUCE_ACK
• INTRODUCE1
• INTRODUCE2
• ESTABLISH_RENDEZVOUS
• RENDEZVOUS_ESTABLISHED
• RENDEZVOUS1
• RENDEZVOUS2

Encoding and decoding for these messages should all be implemented in tor-cell, as should encoding and decoding for any of their inner encrypted parts. But tor-cell should not actually handle the cryptography for these cells; our regular architecture would say that happened at a different level.

To get examples of these cells for testing, run a chutney network with Tor instrumented to dump all of its cells.

We should have a good answer here about #525 (closed) before we get too deeply involved.