Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_add.c

@@ -0,0 +1,29 @@
+#include "tommath_private.h"
+#ifdef MP_ADD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* high level addition (handles signs) */
+mp_err mp_add(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   /* handle two cases, not four */
+   if (a->sign == b->sign) {
+      /* both positive or both negative */
+      /* add their magnitudes, copy the sign */
+      c->sign = a->sign;
+      return s_mp_add(a, b, c);
+   }
+
+   /* one positive, the other negative */
+   /* subtract the one with the greater magnitude from */
+   /* the one of the lesser magnitude. The result gets */
+   /* the sign of the one with the greater magnitude. */
+   if (mp_cmp_mag(a, b) == MP_LT) {
+      MP_EXCH(const mp_int *, a, b);
+   }
+
+   c->sign = a->sign;
+   return s_mp_sub(a, b, c);
+}
+
+#endif