code>, <code>mpq_sub</code>: The use of
     <code>mpz_mul(x,y,x)</code> causes temp allocation or copying in
     <code>mpz_mul</code> which can probably be avoided.  A rewrite using
     <code>mpn</code> might be best.
<li> <code>mpn_gcdext</code>: Don't test <code>count_leading_zeros</code> for
     zero, instead check the high bit of the operand and avoid invoking
     <code>count_leading_zeros</code>.  This is an optimization on all
     machines, and significant on machines with slow
     <code>count_