mpf</code>: Detect exponent overflow and raise some exception.
     It'd be nice to allow the full <code>mp_exp_t</code> range since that's
     how it's been in the past, but maybe dropping one bit would make it
     easier to test if e1+e2 goes out of bounds.
</ul>



<h4>Machine Independent Optimization</h4>
<ul>
<li> <code>mpf_cmp</code>: For better cache locality, don't test for low zero
     limbs until the high limbs fail to give an ordering.  Reduce code size by
     turning the three <code>mpn_cmp<