Improve homing by checking endstop state from the stepping ISRs.

This removes the delay between triggering of the endstop and the end of
movement, as was originally present due to the use of PinWatcher.
Additionally, homing now doesn't depend on the presence of pin change
interrupts, or any particular mapping between pins and pin change
interrupt pin numbers.
Remove AvrPinWatcher, it is no longer needed.