Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_zero.c

@@ -0,0 +1,13 @@
+#include "tommath_private.h"
+#ifdef MP_ZERO_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* set to zero */
+void mp_zero(mp_int *a)
+{
+   a->sign = MP_ZPOS;
+   s_mp_zero_digs(a->dp, a->used);
+   a->used = 0;
+}
+#endif