User channel memberships are actually a set of memberships, not a single
value. This introduces memberships, a type representing a set of
memberships, stored as an array of memberships ordered by descending
rank.
This also adds multi-prefix to the permanent downstream and upstream
capabilities, so that we try to get all possible channel memberships.
- Add RPL_ISUPPORT support with CHANMODES, CHANTYPES, PREFIX parsing
- Add support for channel mode state with mode arguments
- Add upstream support for RPL_UMODEIS, RPL_CHANNELMODEIS
- Request channel MODE on upstream channel JOIN
- Use sane default channel mode and channel mode types
