Function switch_map 

pub fn switch_map<T, U, S, Inner, F>(f: F, s: S) -> impl Stream<Item = U>
where
    S: Stream<Item = T> + Unpin,
    Inner: Stream<Item = U> + Unpin,
    F: Fn(T) -> Inner,

Switch to new inner stream on each outer value, cancelling previous. Runtime-agnostic using futures::select!