Stepper: remove more redundant fields
Stepper: various cleanups in preparation for Robot::actuators to become Actuator* rather than StepperMotor*, also fix deceleration on premature stop
StepTicker: remove some redundant fields
StepTicker: remove redundant active_motors list which simply mirrors stepper_motors
StepTicker: Use Timer0->MR1 to unstep instead of Timer1
StepTicker: Whitespace change only. Separate commit to vastly improve readability of d121fd's diff
StepTicker: use constants for register manipulation to improve readability
StepTicker: move timer event into class member to reduce constant dereference of singleton pointer
StepTicker: whitespace change. Separate commit to improve readability of c1fd61's diff