The Magic Wormhole protocols

The protocols were originally developed alongside the Python implementation in a common repository. But now, as there are multiple implementations, discussion of the protocols should be separated into their own repository.

Game plan

The 1.0 release simply mirrors the "status quo", as rendered on https://magic-wormhole.readthedocs.io/en/latest/. In succedent minor updates, the Python-specific parts will get removed. There are a lot of minor things that could be clarified or some examples added. All modifications should be as compatible as possible to the original text. Sadly, this isn't always possible as the original text may contain contradictions that must be resolved.