Add merge_round_robin for StreamExt

[RustyNova016]

This function allows merging two streams using a custom "round robin" policy.

This is an useful alternative to chain as the second stream doesn't wait for the first to finish, and both can be interweaved toghether

Example:

use futures::stream::{self, StreamExt};
let stream1 = stream::iter(vec!["a", "a"]);
let stream2 = stream::iter(vec!["b", "b", "b", "b", "c"]);
let stream = stream1.merge_round_robin(stream2, 1, 2);
let result: Vec<_> = stream.collect().await;
assert_eq!(result, vec![
    "a",
    "b",
    "b",
    "a",
    "b",
    "b",
    "c"
]);

I am quite new to contributing to the official channels, so if there's anything wrong, please tell!

[taiki-e]

Thanks for the PR. What is the difference compared to stream::select and stream::select_with_strategy?

https://docs.rs/futures/latest/futures/stream/fn.select.html
https://docs.rs/futures/latest/futures/stream/fn.select_with_strategy.html#round-robin

[RustyNova016]

Well... I admit I didn't see that stream::select existed.

But there's a couple changes:

• It's not a standalone function, and is part of StreamExt. So it can be inlined
• The strategy is easier to implement. If we want to pass 2 items from stream A and 1 item from stream B, we only have to set the numbers. Doing it with select_with_strategy would require either storing state in a mutex or use enumerate and modulos

But I understand if it feels too specific when we already have something else implemented