Use `Listener<P>` instead of `Listener<Data = P>`

[plohan]

The current trait use associate type to correlate between listener and intermediate payload.

pub trait Listener: Send + Sync {
    type Data;
    type S: Stream<Item = Self::Data> + Unpin + Send;

    fn into_stream(self) -> Self::S;
}

This is suboptimal design. For example, let's say that raw payload from listener L can be turn into intermediate payload P (Data = P). Which mean

impl Listener for L {
  type Data = P;
  ...
}

Then, with this approach, it is not possible to make that listener to turn into other intermediate payload.

pub trait Listener<P>: Send + Sync {
    type S: Stream<Item = P> + Unpin + Send;

    fn into_stream(self) -> Self::S;
}

With this implementation, it is possible to implement many intermediate payload for one type of listener.

impl Listener<P1> for L {
  ...
}
impl Listener<P2> for L {
  ...
}