Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_init_copy.c

@@ -0,0 +1,21 @@
+#include "tommath_private.h"
+#ifdef MP_INIT_COPY_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* creates "a" then copies b into it */
+mp_err mp_init_copy(mp_int *a, const mp_int *b)
+{
+   mp_err     err;
+
+   if ((err = mp_init_size(a, b->used)) != MP_OKAY) {
+      return err;
+   }
+
+   if ((err = mp_copy(b, a)) != MP_OKAY) {
+      mp_clear(a);
+   }
+
+   return err;
+}
+#endif