* Exploit restricted range for inputs (e.g. fixnums).
* When the divisor is even, simplify with a mask instead of a shift.
* Clean up the code a bit: we don't want modular arithmetic, it's
actually all guaranteed not to wrap around.
* Change the test so that larger divisors are a bit more likely to
get tested too.
* Lots more things can be done, including generalizing the transform
to pretty much arbitrary rational divisor, or avoiding the costly
general code sequence in nearly all cases. Unfortunately, it's a
lot of (somewhat original, at that) code, and can be fairly slow;
if it matters to someone, I can try and find a compromise (contrib?).