Function subscribe 

pub fn subscribe<M, I, St, Si>(
    stream: St,
    control_channel: Si,
) -> (MessageStream<St, M>, Subscription<Si, M, I>)
where
    M: Message,
    St: Stream<Item = M>,
    Si: Sink<I>,

Wrap a stream and an associated control channel into a connected (MessageStream, Subscription) pair, in which the subscription can be used to send and receive control messages over the stream.