Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/s_mp_add.c

@@ -0,0 +1,70 @@
+#include "tommath_private.h"
+#ifdef S_MP_ADD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* low level addition, based on HAC pp.594, Algorithm 14.7 */
+mp_err s_mp_add(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   int oldused, min, max, i;
+   mp_digit u;
+   mp_err err;
+
+   /* find sizes, we let |a| <= |b| which means we have to sort
+    * them.  "x" will point to the input with the most digits
+    */
+   if (a->used < b->used) {
+      MP_EXCH(const mp_int *, a, b);
+   }
+
+   min = b->used;
+   max = a->used;
+
+   /* init result */
+   if ((err = mp_grow(c, max + 1)) != MP_OKAY) {
+      return err;
+   }
+
+   /* get old used digit count and set new one */
+   oldused = c->used;
+   c->used = max + 1;
+
+   /* zero the carry */
+   u = 0;
+   for (i = 0; i < min; i++) {
+      /* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
+      c->dp[i] = a->dp[i] + b->dp[i] + u;
+
+      /* U = carry bit of T[i] */
+      u = c->dp[i] >> (mp_digit)MP_DIGIT_BIT;
+
+      /* take away carry bit from T[i] */
+      c->dp[i] &= MP_MASK;
+   }
+
+   /* now copy higher words if any, that is in A+B
+    * if A or B has more digits add those in
+    */
+   if (min != max) {
+      for (; i < max; i++) {
+         /* T[i] = A[i] + U */
+         c->dp[i] = a->dp[i] + u;
+
+         /* U = carry bit of T[i] */
+         u = c->dp[i] >> (mp_digit)MP_DIGIT_BIT;
+
+         /* take away carry bit from T[i] */
+         c->dp[i] &= MP_MASK;
+      }
+   }
+
+   /* add carry */
+   c->dp[i] = u;
+
+   /* clear digits above oldused */
+   s_mp_zero_digs(c->dp + c->used, oldused - c->used);
+
+   mp_clamp(c);
+   return MP_OKAY;
+}
+#endif