Method iter_mut

publication/code/chapter04/stack.rs:73–80

(&mut self)

    }

    fn iter_mut(&mut self) -> IterMut<T> {
        let mut iterator = IterMut { stack: Vec::new() };
        for item in self.data.iter_mut() {
            iterator.stack.push(item);
        }

        iterator
    }
}

// 实现三种迭代功能