Adjust async API so HeaderReader and OggStreamReader can be chained

[tuxmark5]

It should be possible to write something like this:

let stream = HeadersReader::new(data_reader)
  .and_then(|(reader, headers)| OggStreamReader::new(reader, headers))
  .flatten_stream();

#[async]
for packet_block in stream {
  for packet in packet_block {
    stream_yield!(packet);
  }
}

Right now this is not possible, as and_then takes the ownership of HeadersReader. This would be fine if not for OggStreamReader, which requires passing the original HeadersReader to it.

So HeadersReader Future should return not only header set, but also itself or some other object that can be given to OggStreamReader to continue reading the ogg file.