Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_dr_reduce.c

@@ -0,0 +1,68 @@
+#include "tommath_private.h"
+#ifdef MP_DR_REDUCE_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
+ *
+ * Based on algorithm from the paper
+ *
+ * "Generating Efficient Primes for Discrete Log Cryptosystems"
+ *                 Chae Hoon Lim, Pil Joong Lee,
+ *          POSTECH Information Research Laboratories
+ *
+ * The modulus must be of a special format [see manual]
+ *
+ * Has been modified to use algorithm 7.10 from the LTM book instead
+ *
+ * Input x must be in the range 0 <= x <= (n-1)**2
+ */
+mp_err mp_dr_reduce(mp_int *x, const mp_int *n, mp_digit k)
+{
+   mp_err err;
+
+   /* m = digits in modulus */
+   int m = n->used;
+
+   /* ensure that "x" has at least 2m digits */
+   if ((err = mp_grow(x, m + m)) != MP_OKAY) {
+      return err;
+   }
+
+   /* top of loop, this is where the code resumes if
+    * another reduction pass is required.
+    */
+   for (;;) {
+      int i;
+      mp_digit mu = 0;
+
+      /* compute (x mod B**m) + k * [x/B**m] inline and inplace */
+      for (i = 0; i < m; i++) {
+         mp_word r         = ((mp_word)x->dp[i + m] * (mp_word)k) + x->dp[i] + mu;
+         x->dp[i]  = (mp_digit)(r & MP_MASK);
+         mu        = (mp_digit)(r >> ((mp_word)MP_DIGIT_BIT));
+      }
+
+      /* set final carry */
+      x->dp[i] = mu;
+
+      /* zero words above m */
+      s_mp_zero_digs(x->dp + m + 1, (x->used - m) - 1);
+
+      /* clamp, sub and return */
+      mp_clamp(x);
+
+      /* if x >= n then subtract and reduce again
+       * Each successive "recursion" makes the input smaller and smaller.
+       */
+      if (mp_cmp_mag(x, n) == MP_LT) {
+         break;
+      }
+
+      if ((err = s_mp_sub(x, n, x)) != MP_OKAY) {
+         return err;
+      }
+   }
+   return MP_OKAY;
+}
+#endif