ported by Larry
     Lambe.
<li> <code>mpq_add</code>, <code>mpq_sub</code>: The gcd fits a single limb
     with high probability and in this case <code>modlimb_invert</code> could
     be used to calculate the inverse just once for the two exact divisions
     "op1.den / gcd" and "op2.den / gcd", rather than letting
     <code>mpn_divexact_1</code> do it each time.  This would require a new
     <code>mpn_preinv_divexact_1</code> interface.  Not sure if it'd be worth
     the trouble.
<li> <code>mpq_add</