Renamed laser-related variables.  lm -> pm, pd -> ps, pl -> pd.
Formatting only.
Cosmetic only.
More to do.
Now uses motor functions instead of poking registers directly.
Interim checkin -- laser pulse code roughed in, but more to write.
Added timers' TCCRnC register and FOCnx bit definitions.
Made explicit which pulse modes with with which operations (Dwell, Cut, Move).
Added assertions to type-check get_*_variable().
No longer export get_engine_state().
Cleaned up init_lasers(); corrected OCRxn initialization.
Removed unhelpful comment.
There is more to do.
Added -fshort-enums so enumerated types are 8 bits.
Corrected the fault generated on a missed interrupt to F_SI.
Removed unneeded inclusion of <stdio.h>.
Deprecated non_P printf w/ compilation error.
No longer export get_engine_state().
Scheduler in progress.  Laser pulses mostly coded.
Laser scheduling: OFF before ON, all mode implemented except continued trains.