Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/s_mp_zero_digs.c

@@ -0,0 +1,23 @@
+#include "tommath_private.h"
+#ifdef S_MP_ZERO_DIGS_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_digs(mp_digit *d, int digits)
+{
+#ifdef MP_USE_MEMOPS
+   if (digits > 0) {
+      memset(d, 0, (size_t)digits * sizeof(mp_digit));
+   }
+#else
+   while (digits-- > 0) {
+      *d++ = 0;
+   }
+#endif
+}
+
+#endif