WIP: Support for msc3946

[DMRobertson]

Closes #38

Utterly WIP. I'm opening the PR for better visibility of in-flight work.

Concerns:

• What is there to stop me from marking rooms A and B as both superseding room C? At present the latest seen predecessor event wins.
• If I mark room A as superseding B1, then mark A as superseding B2, is B1 still considered to be replaced by A?
• What do we do if we learn that A supersedes B but we have no knowledge of room B? It would be a shame if we later joined B and didn't present it as being replaced by B.

[DMRobertson]

Going to drop this. I didn't get far, it's not a priority, and there looks to have been some bitrot.