Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/s_mp_sub.c

@@ -0,0 +1,56 @@
+#include "tommath_private.h"
+#ifdef S_MP_SUB_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
+mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   int oldused = c->used, min = b->used, max = a->used, i;
+   mp_digit u;
+   mp_err err;
+
+   /* init result */
+   if ((err = mp_grow(c, max)) != MP_OKAY) {
+      return err;
+   }
+
+   c->used = max;
+
+   /* set carry to zero */
+   u = 0;
+   for (i = 0; i < min; i++) {
+      /* T[i] = A[i] - B[i] - U */
+      c->dp[i] = (a->dp[i] - b->dp[i]) - u;
+
+      /* U = carry bit of T[i]
+       * Note this saves performing an AND operation since
+       * if a carry does occur it will propagate all the way to the
+       * MSB.  As a result a single shift is enough to get the carry
+       */
+      u = c->dp[i] >> (MP_SIZEOF_BITS(mp_digit) - 1u);
+
+      /* Clear carry from T[i] */
+      c->dp[i] &= MP_MASK;
+   }
+
+   /* now copy higher words if any, e.g. if A has more digits than B  */
+   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_SIZEOF_BITS(mp_digit) - 1u);
+
+      /* Clear carry from T[i] */
+      c->dp[i] &= MP_MASK;
+   }
+
+   /* clear digits above used (since we may not have grown result above) */
+   s_mp_zero_digs(c->dp + c->used, oldused - c->used);
+
+   mp_clamp(c);
+   return MP_OKAY;
+}
+
+#endif