* Support helical interpolation
* Optimize arc target detection code utilizing the primary axis of travel
* Arcs might be a step or two off because of FP gotchas. Must add a little nudge in the end there?
* Support inverse feed rate for arcs
* Implement homing cycle in stepper.c
* Implement limit switch support in stepper.c (use port-triggered interrupts?)
* 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.
* Eliminate need for x and y in step_arc_along_
* Tool table
* Tool length offsets
* Tool change M6
* Path Control Modes
* Spindle speed support