om high to low and look at
     each step for values differing by more than the possible carry bit from
     the uncalculated portion.
<li> <code>mpq_cmp</code> could do the same high-to-low progressive multiply
     and compare.  The benefits of karatsuba and higher multiplication
     algorithms are lost, but if it's assumed only a few high limbs will be
     needed to determine an order then that's fine.
<li> <code>mpn_add_1</code>, <code>mpn_sub_1</code>, <code>mpn_add</code>,
     <code>mpn_sub</code>: