Function select_multiple 

pub fn select_multiple<F>(futures: Vec<F>) -> SelectMultiple<F> 
where
    F: Future + Unpin,

Select from multiple futures 从多个future中选择

For Phase 2, this provides a simpler alternative to the full select! macro. 对于第2阶段，这提供了完整的select!宏的更简单替代方案。