 the table should just go up to base 16, and have
     code to generate data above that, if and when required.  Naturally this
     assumes the code would be smaller than the data saved.
<li> <code>__mp_bases</code> field <code>big_base_inverted</code> is only used
     if <code>USE_PREINV_DIVREM_1</code> is true, and could be omitted
     otherwise, to save space.
<li> <code>mpz_get_str</code>, <code>mtox</code>: For power-of-2 bases, which
     are of course fast, it seems a little silly to make a second 