Concurrent.pmod

Previously code such as:

  Concurrent.Future f2 = Concurrent.results(({}));

  f2->on_success(lambda(array(string) a) { werror("success: %O.\n", a); })
    ->on_failure(lambda(mixed err) { werror (describe_backtrace(err)); });

... would lead to a backtrace because the future was destructed prematurely.