New stepper subsystem bug fixes.

- New stepper algorithm with the new optimized planner seems to be
working nearly twice as fast as the previous algorithm.

- For one, the planner computation overhead is probably a fraction of
what it used to be with the worst case being about half still.

- Secondly, anytime the planner plans back to the first executing
block, it no longer overwrites the block conditions and allows it to
complete without lost steps. So no matter if the streams slows, the
protected planner should keep the steppers moving without risk of lost
steps (although this still needs to be tested thoroughly and may
audibly sound weird when this happens.)

- It now seems that the bottleneck is the serial baudrate (which is
good!)