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].
dda: Move axis calculations into loops.

Clean up code to reduce duplication by consolidating code into
loops for per-axis actions.
home: Reduce duplication with axis arrays
datalog: show traced data as signed ints, not unsigned
Rename confusing variable name

'all_time' sounds like forever to me, but this variable really
tracks the last time we hit one of "all the axes".  It sticks
out more now in looping, so rename it to make sense.
Trivial cleanups

Move some code around, fix some formatting, and hide a couple
of variables when they're not being used.
Notes.