Moved back to a fork of ruma with timo's key-backup and cross-signing
branch. Ephemeral events in sync responses are EphemeralRoomEventStub
(they also have no room_id like all of sync responses events)
When creating some responses (sync) an AnyRoomEventStub is needed for this
PduEvent will deserialize the JSON as a Stub event and a non stub event
when needed. Ephemeral and account events are checked to be the correct
type and filtered out if not. This requires an extra `deserialize` call
which could/should be removed.
TODO: Possibly get rid of EventJson in some places.