y
     fast on one or two limb moduli, due to a lot of function call
     overheads.  These could perhaps be handled as special cases.
<li> <code>mpz_powm</code> and <code>mpz_powm_ui</code> want better
     algorithm selection, and the latter should use REDC.  Both could
     change to use an <code>mpn_powm</code> and <code>mpn_redc</code>.
<li> <code>mpz_powm</code> REDC should do multiplications by <code>g[]</code>
     using the division method when they're small, since the REDC form of a
     small mul