Coding Matrix in Rust | 2023-04-17

This time: * We managed to get Arnie to only send messages into channels that are suitable for that message * We merged and then reverted part of Jonas' PR to make `filter_n` a nicer function * We started thinking about how to get Arnie to send read receipts: it's going to push us to make the refactor we talked about before: stop passing in a callback, and instead just pass a channel to push outgoing messages through.