updated todo

todo.txt

@@ -1,7 +1,6 @@
-* Support helical interpolation
+* Use errno to detect fp-errors
 * 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