Add join_all function for waiting on an iterator of futures

[surechen]

Hi, everyone. Thank you for reading this issue. The crate futures has a function futures::future::join_all. But the performance of this function in futures seem not very good. Tokio has talked about this. Will async_std provide this function? I'm willing to work on this. If I add a vec marks in JoinAll to record ready futures to prevent poll them again, can solve this performance problem?

//struct JoinAll looks like this

pin_project! {
    #[allow(missing_docs)]
    #[allow(missing_debug_implementations)]
    pub struct JoinAll<F>
    where
        F: Future,
    {
        #[pin] elems: Box<[MaybeDone<F>]>,
        #[pin] marks: Box<Vec<u8>>,
    }
}

// test

async_std::task::block_on(async {
    use async_std::prelude::*;
    use async_std::future;
    let futurevec = vec![future::ready(5u8), future::ready(6u8)];
    let join_all = future::ready(0).join_all(futurevec);
    let result = join_all.await;
    let mut count = 0;
    for i in result.iter() {
        println!("test count:{},result:{}", count, i);
        count += 1;
    }
    assert_eq!(result, [5u8, 6u8]);
});