Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_rand_source.c

@@ -0,0 +1,12 @@
+#include "tommath_private.h"
+#ifdef MP_RAND_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+mp_err(*s_mp_rand_source)(void *out, size_t size) = s_mp_rand_platform;
+
+void mp_rand_source(mp_err(*source)(void *out, size_t size))
+{
+   s_mp_rand_source = (source == NULL) ? s_mp_rand_platform : source;
+}
+#endif