Function ls_print 

fn ls_print<W: Write>(
    out: &mut W,
    fs: &FileSystem<Sha256HashValue>,
    dir: &Directory<Sha256HashValue>,
    path: &[u8],
    seen_leaf_ids: &mut HashSet<LeafId>,
    filter: Option<&[String]>,
) -> Result<()>

Walk and print entries: directory line first, then recurse into children.