Function streaming_parallel_map 

pub fn streaming_parallel_map<T, R, F>(
    items: &[T],
    f: F,
) -> impl Iterator<Item = R>
where
    T: Sync + Clone,
    R: Send + 'static,
    F: Fn(&T) -> R + Send + Sync + 'static,

Streaming parallel map (process as results come)