Here's a draft of multistream-2.0 (+ a retrospective that you can skip).

1. Split retro into a separate file.
2. Add an example.
3. Rename hello to advertise. I'd like to encourage using many tiny protocols
instead of adding more and more features to bloated protocols. That means
separate protocol advertisement, hello, etc. protocols. Maybe this is going too
far and we should just call this "identify".
4. Rename serial-multiplex to serial-stream and multistream/choose to
speculative-stream (and move it out of the multistream protocol family).
5. Use more varints. Really, we can probably go a step further and make
serial-multiplex use a varint.

* use -> multicodec
* dynamic -> string
* contextual -> dynamic

TODO: Move this elsewhere. It's not a part of multistream and is only relevant
because it came up in the retro.