Move clock stuff from timer.c/.h to clock.c/.h.

I guess that's where it belongs.
heater.c: add a hysteresis when using BANG_BANG.

Before, the heater would turn off and on at each tiny temperature
fluctuation. Or better: at every tiny analog voltage fluctuation.
config.*.h: distribute changed TEMP_HYSTERESIS description to all templates.
Testcases: add nothing.gcode.

This is for running stuff with run_in_simulavr.sh which needs no
G-code.
config_wrapper.h: protect against USB/serial misconfiguration.
dda.c: some whitespace cleaning.
dda.c: file attempt to use sign of dda->delta_um[] to set direction.
CoreXY, dda.c: start by making steps an array.

Goal is to calculate steps in a separate function to allow different
methods of steps calculation, which is neccessary for supporting
different kinematics types. Accordingly we have to calculate steps
for all axes before setting directions and such stuff.
CoreXY, dda.c: separate bot axes from extruder axes.
CoreXY, dda.c: calculate delta steps separately.

This will allow us to use it's sign to set direction.

No binary size increase, optimizer can handle this.
CoreXY, dda.c: use our now signed delta_steps to set direction.

This nicely makes the binary 36 bytes smaller.
CoreXY, dda.c: use signed delta_steps to simplify dda->delta_um[] calc.

Saves another 8 bytes binary size.
CoreXY: separate out bot axes distance calculations.

This also introduces dda_kinematics.c/.h and a KINEMATICS definition,
which allows to do different distance calculations depending on the
bot kinematics in use. So far only KINEMATICS_STRAIGHT, which matches
what we had before, but other kinematics types are present in
comments already.
CoreXY: distribute new KINEMATICS definition to all config.h templates.
CoreXY, dda_kinematics.c/.h: introduce KINEMATICS_COREXY.

Implementation isn't that difficult, should be complete and working
fine already.
CoreXY: tell config.h templates about KINEMATICS_COREXY.
dda.c: abs32() is obsolete now.

It's obsolete, because it's used for signed ints, only.
dda_lookahead.c: loop up prevF calculation.

This saves a whopping 98 bytes binary size.
dda_lookahead.c: loop up currF, too.

Even more savings this time, 146 bytes binary size.
dda_lookahead.c: put MAX_JERK_{XYZE} into an array.

This costs 88 bytes binary size for now, but will us allow to
loop up a nice section of code.