Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/s_mp_zero_buf.c

@@ -0,0 +1,22 @@
+#include "tommath_private.h"
+#ifdef S_MP_ZERO_BUF_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+#ifdef MP_USE_MEMOPS
+#  include <string.h>
+#endif
+
+void s_mp_zero_buf(void *mem, size_t size)
+{
+#ifdef MP_USE_MEMOPS
+   memset(mem, 0, size);
+#else
+   char *m = (char *)mem;
+   while (size-- > 0u) {
+      *m++ = '\0';
+   }
+#endif
+}
+
+#endif