MotionPlanner: don't allow and check for motionless commands.
We can do the test in PrinterMain more efficiently using total_steps.
MotionPlanner: optimize feeding of commands.
Remove the InputCommand struct and have the user code directly write to
SplitBuffer.
MotionPlanner: split commandDone() into axesCommandDone() and
channelCommandDone().
MotionPlanner: always call work() in the FastEvent handler.
Instead of also calling it from the xxxCommandDone(). This should reduce
stack usage.
aprinter-ramps13.cpp: make use of the available RAM.
Getting much closer do stack/heap collision. Reduce the stepper buffers
a bit in favor of lookahead buffers.