Only expose config settings via methods on the Config struct. This
allows default value handling to live entirely inside the config code,
solves various borrowing issues with the "create default bot config
value" solution, and allows us to avoid cloning the bot config values.
The downside is that the config must now be in an Arc since its
ownership is shared in multiple places and the matrix SDK requires
thread-safe types (perhaps in the future we re-compose traits and use
Rc for config instead).
This commit also further cleans up and splits up the bot code for the
matrix connection, notably making the main message event handler
smaller by splitting out the "should we process the message" checks
into a separate function.
Instead of using an Arc Mutex for state management embedded directly
into the bot, utilize actor pattern, with the idea that this will be
much more useful than simply logging a message once in the future.
This also refactors the bot code so that instead of a single run_bot
function, the DiceBot struct now has a run() method attached to it.
This also necessitated changes and cleanup to the dicebot main, which
is for the better anyhow.
The error and config types are also now in their own files, and
implemented for more in-depth use cases.