* Implement homing cycle in stepper.c
* Implement limit switch support in stepper.c (use port-triggered interrupts?)
* How to implement st_set_pace? Consider synchronizing when pace is changed
* How on earth am I going to deal with arcs in setups that have different steps/mm on each axis? Must 
  support elipses?! Oh no.
* Support helical interpolation
* Eliminate need for circle_x and circle_y in step_arc_along_
* Tool table
* Tool length offsets
* Tool change M6
* Path Control Modes
* Spindle speed support