Initial (non-working) IRC server/s2c code

- A new macro, sink_outbox!, was added to deduplicate the same
  "send messages into this futures sink from this outbox array"
  behaviour used in multiple places now.
- We now have the beginnings of an IRC server / s2c protocol impl!
  It handles client registration, and pretty much nothing else
  after that at this stage - but it's getting along!