avr-asm-ops/div_11_16_l15_s13.h: fix cycle count
AxisStepper: compute discriminant in the ISR.
This saves a lot of RAM and allows a much larger stepper buffer,
at the expense of worse worst case ISR time.
aprinter/printer/aprinter-melzi.cpp: adjust to my liking after some
testing with Melzi
aprinter/printer/aprinter-rampsfd.cpp: adjust defaults to my liking