Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_mulmod.c

@@ -0,0 +1,15 @@
+#include "tommath_private.h"
+#ifdef MP_MULMOD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* d = a * b (mod c) */
+mp_err mp_mulmod(const mp_int *a, const mp_int *b, const mp_int *c, mp_int *d)
+{
+   mp_err err;
+   if ((err = mp_mul(a, b, d)) != MP_OKAY) {
+      return err;
+   }
+   return mp_mod(d, c, d);
+}
+#endif