New mpint function mp_get_integer().

If you have an mp_int that you know will fit in an ordinary integer
type, this function gives it to you in that form.

mpint.c

@@ -257,6 +257,18 @@ unsigned mp_get_bit(mp_int *x, size_t bit)
                 (bit % BIGNUM_INT_BITS));
 }
 
+uintmax_t mp_get_integer(mp_int *x)
+{
+    uintmax_t toret = 0;
+    for (size_t i = x->nw; i-- > 0 ;) {
+        /* Shift in two stages to avoid undefined behaviour if the
+         * shift count equals the integer width */
+        toret = (toret << (BIGNUM_INT_BITS/2)) << (BIGNUM_INT_BITS/2);
+        toret |= x->w[i];
+    }
+    return toret;
+}
+
 void mp_set_bit(mp_int *x, size_t bit, unsigned val)
 {
     size_t word = bit / BIGNUM_INT_BITS;