RoNode: read-only parallelization with minimal overhead

Introducing a rayon-compatible primtive, RoNode, which allows for parallel read-only document scans. See #53 for details

Example use:

fn dfs_parallel_count(node: RoNode) -> i32 {
  1 + node
    .get_child_nodes()
    .into_par_iter()
    .map(dfs_parallel)
    .sum::<i32>()
}