MotionPlanner: workaround gcc performance bug.
Will be useful when we enable 24-bit integers, as the size of the
AxisStepper Command stucture will become 12 then. See:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59174
AxisStepper: save some RAM.
Do the v0 calculation in the ISR.
Force some inlining, needed for new gcc.