DDA: convert um_to_steps_* to generic implementation.

A generic implementation here will allow callers to pass the
target axis in as a parameter so the callers can also be made more
generic.

Traumflug notes:

Split out application of the new implementation in dda.c into its
own commit.

This actually costs 128 bytes, but as we can access axes from within
a loop now, I expect to get more savings elsewhere.

Interestingly, binary size is raised by another 18 bytes if

  um_to_steps(int32_t, enum axis_e)

is changed to

  um_to_steps(enum axis_e, int32_t)

even on the 8-bit ATmega. While putting the axis number to the
front might be a bit more logical (think of additional parameters,
the axis number position would move), NXP application note
AN10963 states on page 10ff, 16-bit data should be 16-bit aligned
and 32-bit data should be 32-bit aligned for best performance.
Well, so let's do it this way.
DDA: use new generic um_to_steps_* in dda_new_startpoint().

This was contributed by Phil Hord as part of another commit.

It saves 168 bytes, to it more than outweights the overhead of
introducing a generic implementation already.
DDA: Move axis calculations into loops, part 1.

Clean up code to reduce duplication by consolidating code into
loops for per-axis actions.

Traumflug notes:

Split this once huge commit into smaller ones for ease of
reviewing and bisecting (in case something went wrong).

Part 1 is to put dda_create() distance calculations into loops.
This reduces binary size by another whopping 756 bytes.
dda_maths.h: remove now obsolete um_to_steps_[xyze].