also clean up a bit
  import the defmt macros and use them without the prefix defmt::
    make the debug print levels a bit more consistent
      trace for tracking where execution goes, debug for actual data
    adjust DEFMT_LOG to debug (in shell.nix)
  remove the panic-halt dependency (no longer needed with panic-probe)
  set opt-level 1 for debug builds so they still fit in the chip