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.