st3m: implement modality

This change makes st3m aware of what Micropython is doing, and lets st3m
provide some 'failsafe' functionality even if Micropython crashes.

Most notably, this lets us draw some useful messages when the badge is
in REPL mode, and lets us implement things like restarting the badge via
buttons if that happens.

This API isn't fully used yet, but this is a start.