Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_init_set.c

@@ -0,0 +1,16 @@
+#include "tommath_private.h"
+#ifdef MP_INIT_SET_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* initialize and set a digit */
+mp_err mp_init_set(mp_int *a, mp_digit b)
+{
+   mp_err err;
+   if ((err = mp_init(a)) != MP_OKAY) {
+      return err;
+   }
+   mp_set(a, b);
+   return err;
+}
+#endif