FuturesOrdered push_front does not work at the start

[conradludgate]

Running this code:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=30c5f139539ccf5dd237299c88b56a2e

use std::future::ready;
use std::collections::VecDeque;
use futures::stream::{FuturesOrdered, StreamExt};

#[tokio::main]
async fn main() {
    let mut set = VecDeque::new();

    set.push_back(2);  // should be the last
    set.push_front(1); // should be the second
    set.push_front(0);  // should be the first
    
    let x: Vec<i32> = set.into_iter().collect();
    assert_eq!(x, [0, 1, 2]);

    let mut set = FuturesOrdered::new();
    
    set.push_back(ready(2));  // should be the last
    set.push_front(ready(1)); // should be the second
    set.push_front(ready(0));  // should be the first
    
    let x: Vec<i32> = set.collect().await;
    assert_eq!(x, [0, 1, 2]);
}

You see the assertion on the VecDeque passes, but the same assertion through FuturesOrdered fails.

I noticed this working on my own implementation in https://github.com/conradludgate/futures-buffered - I found that turning the usize counters into isize fixes it with no noticeable downsides.