instead of testing for special field primes each time in mpmod(),
make it explicit with a mpfiled() function that tests a modulus N
to be of some special form that can be reduced more efficiently with
some precalculation, and replaces N with a Mfield* when it can. the
Mfield*'s are recognized by mpmod() as they have the MPfield flag
set and provide a function pointer that executes the fast reduction.
