Onion service APIs, part 1.
This branch sketches out lower-level apis that will be needed for onion services, in:
- tor-hscrypto (new crate)
- tor-cell
- tor-netdoc
- tor-cert
It also adds a new document in doc/dev that outlines the protocol.
Obviously these aren't complete, so we'll have to fill in a lot of TODO hs
code. But let's discuss what we have here to make sure we have the general ideas right.