Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_error_to_string.c

@@ -0,0 +1,29 @@
+#include "tommath_private.h"
+#ifdef MP_ERROR_TO_STRING_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* return a char * string for a given code */
+const char *mp_error_to_string(mp_err code)
+{
+   switch (code) {
+   case MP_OKAY:
+      return "Successful";
+   case MP_ERR:
+      return "Unknown error";
+   case MP_MEM:
+      return "Out of heap";
+   case MP_VAL:
+      return "Value out of range";
+   case MP_ITER:
+      return "Max. iterations reached";
+   case MP_BUF:
+      return "Buffer overflow";
+   case MP_OVF:
+      return "Integer overflow";
+   default:
+      return "Invalid error code";
+   }
+}
+
+#endif