State should unsubscribe if readonly

[wjian23]

Problem
StateWriter should auto unsubscribe when read only

Example
the follow test should be pass.

#[test]
  fn unsubscribe_when_not_writer() {
    struct Guard {
      drop_cnt: Rc<RefCell<i32>>,
    }
    impl Guard {
      fn _use(&self) {}
    }
    impl Drop for Guard {
      fn drop(&mut self) { *self.drop_cnt.borrow_mut() += 1; }
    }

    fn drop_writer_subscribe<W: StateWriter>(w: W, drop_cnt: Rc<RefCell<i32>>) {
      let guard = Guard { drop_cnt: drop_cnt.clone() };
      let r = w.clone_reader();
      w.modifies().subscribe(move |_| {
        guard._use();
        r.clone_reader();
      });
    }

    let drop_cnt = Rc::new(RefCell::new(0));
    {
      drop_writer_subscribe(Stateful::new(()), drop_cnt.clone());
    };
    AppCtx::run_until_stalled();
    assert_eq!(*drop_cnt.borrow(), 1);

    {
      drop_writer_subscribe(Stateful::new(()).map_writer(|v| v, |v| v), drop_cnt.clone());
    };
    AppCtx::run_until_stalled();
    assert_eq!(*drop_cnt.borrow(), 2);

    {
      drop_writer_subscribe(
        Stateful::new(()).split_writer(|v| v, |v| v),
        drop_cnt.clone(),
      );
    };
    AppCtx::run_until_stalled();
    assert_eq!(*drop_cnt.borrow(), 3);
  }