Cleaned up stepper and planner code.

- Added some compile-time error checking. Will add more in future
pushes to ensure settings are correct and within parameters that won't
break anything.

- Pushed some master branch changes with MEGA pin settings.

- Cleaned up planner code and comments to clarify some of the new
changes. Still much to do here.

- Cleaned up the new stepper code. May need to abstract some of the
segment buffer more to fix the feed holds (and integrate homing into
the main stepper routine). With what's planned, this should make the
stepper algorithm easier to attach other types of processes to it,
where it is now tightly integrated with the planner buffer and nothing
else.