math/mpreduce.[ch]: Extend the domain to all positive integers.

Integers of the form (100...)_2 are now acceptable, at last, now that
I've got a grip on the underlying theory.  (It's somewhat embarrassing
that it's taken so long, given that the algorithm was my own work to
begin with, but it was all rather trial-and-error.)

Negative numbers still don't work, and probably never will.