WIP: Begin moving DDA calculations into loops

Barely got started before I ran out of time.

Need to continue replacing um_to_steps_* everywhere.
Got a few more sections to loop in dda.c
Got some more variables to hunt down for optimization.
Need to reduce math/array duplication in max_tics_per_um_per_2_5 area.
And what happened to x_step_interval?
WIP: Continue moving DDA calculations into loops

Still have some _counters to cleanup, maybe.  The rest looks good.