reduce the chance of
     having to do a reallocate if the <code>mpz_t</code> grows a bit more.
     This could only be an option, since it'd badly bloat memory usage in
     applications using many small values.
<li> <code>mpq</code> functions could perhaps check for numerator or
     denominator equal to 1, on the assumption that integers or
     denominator-only values might be expected to occur reasonably often.
<li> <code>count_trailing_zeros</code> is used on more or less uniformly
     distributed nu