Function merge_map_with_options 

pub fn merge_map_with_options<TIn: Clone + 'static, TOut: 'static>(
    project: impl Fn(&TIn) -> Node<TOut> + 'static,
    opts: MergeMapOptions,
) -> Operator<TOut>

merge_map_with_options: merge_map plus optional in-flight inner limiting.

concurrent = Some(n) means at most n live inner deps are subscribed at once; excess outer DATA values are queued and projected lazily when an inner COMPLETE frees a slot.