fx: update fx_iterator to return pointers to values

fx/test/unicode-strings.c

@@ -20,7 +20,7 @@ int main(void)
 	fx_foreach(val, it)
 	{
 		const char *tok = NULL;
-		fx_value_get_cstr(&val, &tok);
+		fx_value_get_cstr(val, &tok);
 		printf("%s\n", tok);
 	}
 	fx_iterator_unref(it);