pz_n_pow_ui</code> does a good job with small bases and stripping
     powers of 2, but it's perhaps a bit too complicated for what it gains.
     The simpler <code>mpn_pow_1</code> is a little faster on small exponents.
     (Note some of the ugliness in <code>mpz_n_pow_ui</code> is due to
     supporting <code>mpn_mul_2</code>.)
     <br>
     Perhaps the stripping of 2s in <code>mpz_n_pow_ui</code> should be
     confined to single limb operands for simplicity and since that's where
     the greatest gai