Function serve 

pub async fn serve<C, F, Fut>(
    stream: DebugRequestStream,
    handler: F,
) -> Result<(), Error>
where
    C: SubCommands,
    F: FnMut(C) -> Fut,
    Fut: Future<Output = Result<String, Error>>,

Serves a DebugRequestStream by parsing commands into C and invoking handler for each command.