Function par_map 

pub fn par_map<R, T, F>(
    reader: &mut FastxReader<R>,
    chunk_size: usize,
    f: F,
) -> Result<Vec<T>>
where
    R: Read,
    T: Send,
    F: Fn(&Sequence) -> T + Send + Sync,

Map every record through f in parallel, preserving input order.