Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_sbin_size.c

@@ -0,0 +1,11 @@
+#include "tommath_private.h"
+#ifdef MP_SBIN_SIZE_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* get the size for an signed equivalent */
+size_t mp_sbin_size(const mp_int *a)
+{
+   return 1u + mp_ubin_size(a);
+}
+#endif