tiplier is normally a full size product.  Probably would need a
     new tuned parameter to say what size multiplier is "small", as a function
     of the size of the modulus.
<li> <code>mpz_powm</code> REDC should handle even moduli if possible.  Maybe
     this would mean for m=n*2^k doing mod n using REDC and an auxiliary
     calculation mod 2^k, then putting them together at the end.
<li> <code>mpn_gcd</code> might be able to be sped up on small to
     moderate sizes by improving <code>find_a</code>, 