A cherry-pick from the kitsune-apigen branch; a family of toJson() and fromJson<>() functions to unify conversion of data back and forth.

Actual usage will come with the next commit.

Kicking and inviting use generated job classes. Rooms in Invite state are stored separately in the hash from those in Join/Leave state because The Spec says so. For clients, this means that the same room may appear twice in the rooms map if it's been left and then the user was again invited to it. The code in Quaternion that properly processes this will arrive shortly.

joinedRoom() and leftRoom() now pass the preempted Invite state of the
room as well; roomMap() only returns Invite and Join rooms, not Leave.

…itions

Notably:
* setJoinState() invocation has been missing from the previous code
* processing invites did not take into account that a Leave state may already exist, thereby forcing clients that display left rooms to look through their records just in case they have to replace a Leave with Invite.
* joinedRoom() was emitted even when the room is not newly joined.