Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_mod.c

@@ -0,0 +1,15 @@
+#include "tommath_private.h"
+#ifdef MP_MOD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* c = a mod b, 0 <= c < b if b > 0, b < c <= 0 if b < 0 */
+mp_err mp_mod(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   mp_err err;
+   if ((err = mp_div(a, b, NULL, c)) != MP_OKAY) {
+      return err;
+   }
+   return mp_iszero(c) || (c->sign == b->sign) ? MP_OKAY : mp_add(b, c, c);
+}
+#endif