d various improvements.
     <ul>
     <li> Make a second seed area within <code>gmp_randstate_t</code> (or
          <code>_mp_algdata</code> rather) to save some copying.
     <li> Make a special case for a single limb <code>2exp</code> modulus, to
          avoid <code>mpn_mul</code> calls.  Perhaps the same for two limbs.
     <li> Inline the <code>lc</code> code, to avoid a function call and
          <code>TMP_ALLOC</code> for every chunk.
     <li> Perhaps the <code>2exp</code> and general LC cases s