Give users a hint in case they obviously forgot to read instructions.
DDA: have an acceleration constant for each axis individually.

For now, keep behaviour identical, like still use STEPS_PER_M_X.
This is about to change soon.
DDA: remember number of the fast axis.
DDA: calculate acceleration for the actual fast axis.

So far we always assumed the fastest axis to have the same steps/mm
as the X axis. In cases where this wasn't true, the movement
wouldn't do sufficient acceleration steps and, accordingly,
not reach the expected maximum speed. This was particularly visible
on a typical Mendel printer, where the Z axis would reach only a
6th of the commanded speed in some configurations.
dda.c: don't bit-shift c0.

While this shifting meant to increase accuracy, there's no actual
use of it, other than that this value gets shifted back and forth.
Let's start to get rid of it.
DDA: get rid of c bit shifting entirely.

To my surprise, this costs about 50 bytes instead of making the
binary smaller.
DDA: replace ACCELERATE_RAMP_LEN with a proper function.

This was (hopefully) the last place where this STEPS_PER_M_X-
assumption was in use.
dda_lookahead.c: get rid of a number of unused functions.

... including the last function which did the STEPS_PER_M_X
assumption.
dda_lookahead.c: note TODO on looping needs.
dda_lookahead.c: do statistics when debugging, only.

This saves a surprising 140 bytes flash and 4 bytes RAM, so
likely a nice little speed enhancement.
dda_lookahead.c: sort more statistics stuff.

Another 40 bytes flash and 8 bytes RAM off.