readdir (async)

ASYNC_DEST_CALL(readdir, callback, dest_path, ...)
  Environment* env = Environment::GetCurrent(args.GetIsolate());
  FSReqWrap* req_wrap;
  char* dest_str = (dest_path);
  int dest_len = dest_str == NULL ? 0 : strlen(dest_str);
  char* storage = new char[sizeof(*req_wrap) + dest_len];
  req_wrap = new(storage) FSReqWrap(env, readdir);
  req_wrap->dest_len(dest_len);
  if (dest_str != NULL)
    memcpy(const_cast<char*>(req_wrap->dest()), dest_str, dest_len + 1);

  int err = uv_fs_readdir(env->event_loop() , &req_wrap->req_, After);

  req_wrap->object()->Set(env->oncomplete_string(), callback);
  req_wrap->Dispatched();
  if (err < 0) {
    uv_fs_t* req = &req_wrap->req_;
    req->result = err;
    req->path = NULL;
    After(req);
  }
  args.GetReturnValue().Set(req_wrap->persistent());