Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_sub.c

@@ -0,0 +1,36 @@
+#include "tommath_private.h"
+#ifdef MP_SUB_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* high level subtraction (handles signs) */
+mp_err mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   if (a->sign != b->sign) {
+      /* subtract a negative from a positive, OR */
+      /* subtract a positive from a negative. */
+      /* In either case, ADD their magnitudes, */
+      /* and use the sign of the first number. */
+      c->sign = a->sign;
+      return s_mp_add(a, b, c);
+   }
+
+   /* subtract a positive from a positive, OR */
+   /* subtract a negative from a negative. */
+   /* First, take the difference between their */
+   /* magnitudes, then... */
+   if (mp_cmp_mag(a, b) == MP_LT) {
+      /* The second has a larger magnitude */
+      /* The result has the *opposite* sign from */
+      /* the first number. */
+      c->sign = (!mp_isneg(a) ? MP_NEG : MP_ZPOS);
+      MP_EXCH(const mp_int *, a, b);
+   } else {
+      /* The first has a larger or equal magnitude */
+      /* Copy the sign from the first */
+      c->sign = a->sign;
+   }
+   return s_mp_sub(a, b, c);
+}
+
+#endif