ec+1 limbs, then it's an in-place division.  If r==u
     and not prec+1 limbs, then move the available limbs up to prec+1 and do
     an in-place there.
<li> <code>mpf_div_ui</code>: Whether the high quotient limb is zero can be
     determined by testing the dividend for high&lt;divisor.  When non-zero, the
     division can be done on prec dividend limbs instead of prec+1.  The result
     size is also known before the division, so that can be a tail call (once
     the <code>TMP_ALLOC</code> is eliminat