Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_reduce_setup.c

@@ -0,0 +1,17 @@
+#include "tommath_private.h"
+#ifdef MP_REDUCE_SETUP_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* pre-calculate the value required for Barrett reduction
+ * For a given modulus "b" it calulates the value required in "a"
+ */
+mp_err mp_reduce_setup(mp_int *a, const mp_int *b)
+{
+   mp_err err;
+   if ((err = mp_2expt(a, b->used * 2 * MP_DIGIT_BIT)) != MP_OKAY) {
+      return err;
+   }
+   return mp_div(a, b, a, NULL);
+}
+#endif