Steppers: simplify
PrinterMain: optimizations
PrinterMain: remove coordinate nan check
For physical axes the clamp will take care of it,
for virtual we're screwed.
PrinterMain: optimization
MotionPlanner: micro-optimization
use last pointer to indicate the new commands list is empty, not first
pointer
MotionPlanner: save a few bytes of RAM
AxisStepper: use less memory on AVR.
Since we can't use a native 24-bit integer due to compiler bugs, hack it
with an array of three chars.
aprinter-ramps13.cpp: make use of the newly freed RAM