This is a version of muldiv() with qn and rn precalculated,
so it can be avoided to re-calclulate it on every instance.
Yet another 116 bytes, unfortunately.
We have multiplies followed by divides all over the place and
most of them are difficult to handle regarding overflows. This
new algorithm handles this fine in all cases, as long as all
three operators and the overall result fits into 32 bits.
Markus Hitter