We may allow more challenge types other than PoW in the future, eg.
captcha. So make the relevent types more generic.
Now the challenge is returned in JSON response as a individual top-level
field `register_challenge` instead of in HTTP headers.
- `Msg` or `msg` is now the canonical term for the substructure in a
room. It includes a `chat` subtype and (in the future) other
administration subtypes like member joining or leaving.
- `Message` or `message` can used in human oriented context like docs
and comments, but only when it is unambiguous.
- `message` is not chosen in code because it's hard to type (at least
for me!), and have ambiguous meaning of:
- "Human readable text" in context of `ApiError`'s field.
- "A unit of data transfer, datagram" in context of WebSocket Message.
- `item` is not chosen because it is overly generic.
