First off, this patch renames `Room::tombstone` to
`Room::tombstone_content` (to be consistent with other methods, such as
`Room::create_content`).
Second, this patch adds the `Room::successor_room` and
`Room::predecessor_room` methods, along with the `SuccessorRoom` and
`PredecessorRoom` types. This naming more or less comes from the Matrix
specification:
- the term _predecessor_ is part of the specification,
- the term _successor_ isn't present _per se_, the words _replacement
room_ are used instead, but I prefer _successor_ as it brings a nice
symmetry with _predecessor_.
Ivan Enderlin
9c227c2321