Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_exch.c

@@ -0,0 +1,13 @@
+#include "tommath_private.h"
+#ifdef MP_EXCH_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* swap the elements of two integers, for cases where you can't simply swap the
+ * mp_int pointers around
+ */
+void mp_exch(mp_int *a, mp_int *b)
+{
+   MP_EXCH(mp_int, *a, *b);
+}
+#endif