Add some Rust utility functions and print support

This gives an indication in the log that Tor was built with Rust
support, as well as laying some groundwork for further string-returning
APIs to be converted to Rust

src/test/test_rust.c

@@ -0,0 +1,31 @@
+/* Copyright (c) 2017, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#include "orconfig.h"
+#include "compat_rust.h"
+#include "test.h"
+#include "util.h"
+
+static void
+test_welcome_string(void *arg)
+{
+  (void)arg;
+  rust_str_t s = rust_welcome_string();
+  const char *c_str = rust_str_get(s);
+  tt_assert(c_str);
+  size_t len = strlen(c_str);
+#ifdef HAVE_RUST
+  tt_assert(len > 0);
+#else
+  tt_assert(len == 0);
+#endif
+
+ done:
+  rust_str_free(s);
+}
+
+struct testcase_t rust_tests[] = {
+  { "welcome_string", test_welcome_string, 0, NULL, NULL },
+  END_OF_TESTCASES
+};
+