Function chunk_writer 

pub fn chunk_writer<W: Write + Seek, F: FnMut(&mut ChunkWriter<W>) -> Result<(), Error>>(
    writer: W,
    chunk_id: ChunkId,
    f: F,
) -> Result<(), Error>

This creates a ChunkWriter and will the closure you passed in with it. This allows you to write contents and children to the ChunkWriter. After you return from the closure this function will make sure that the chunk headers are upates.