gt;2^53 on
     machines with 64-bit <code>mp_exp_t</code>, the precision of
     <code>__mp_bases[base].chars_per_bit_exactly</code> is insufficient and
     <code>mpf_get_str</code> aborts.  Detect and compensate.  Alternately,
     think seriously about using some sort of fixed-point integer value.
     Avoiding unnecessary floating point is probably a good thing in general,
     and it might be faster on some CPUs.
<li> Make the string reading functions allow the `0x' prefix when the base is
     explic