Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_fwrite.c

@@ -0,0 +1,33 @@
+#include "tommath_private.h"
+#ifdef MP_FWRITE_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+#ifndef MP_NO_FILE
+mp_err mp_fwrite(const mp_int *a, int radix, FILE *stream)
+{
+   char *buf;
+   mp_err err;
+   size_t size, written;
+
+   if ((err = mp_radix_size_overestimate(a, radix, &size)) != MP_OKAY) {
+      return err;
+   }
+
+   buf = (char *) MP_MALLOC(size);
+   if (buf == NULL) {
+      return MP_MEM;
+   }
+
+   if ((err = mp_to_radix(a, buf, size, &written, radix)) == MP_OKAY) {
+      if (fwrite(buf, written, 1uL, stream) != 1uL) {
+         err = MP_ERR;
+      }
+   }
+
+   MP_FREE_BUF(buf, size);
+   return err;
+}
+#endif
+
+#endif