possibly just by
     providing an alternate implementation for CPUs with slowish
     <code>count_leading_zeros</code>.
<li> Toom3 could use a low to high cache localized evaluate and interpolate.
     The necessary <code>mpn_divexact_by3c</code> exists.
<li> <code>mpf_set_str</code> produces low zero limbs when a string has a
     fraction but is exactly representable, eg. 0.5 in decimal.  These could be
     stripped to save work in later operations.
<li> <code>mpz_and</code>, <code>mpz_ior</code> and <c