Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_ubin_size.c

@@ -0,0 +1,12 @@
+#include "tommath_private.h"
+#ifdef MP_UBIN_SIZE_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* get the size for an unsigned equivalent */
+size_t mp_ubin_size(const mp_int *a)
+{
+   size_t size = (size_t)mp_count_bits(a);
+   return (size / 8u) + (((size & 7u) != 0u) ? 1u : 0u);
+}
+#endif