Parallel primitives

It is now possible to use llamapun while fully utilizing available CPU cores (configurable as is in rayon).

Most of the examples are now refactored to the parallel primitives, and can see a 20x speedup on high-end chips with 16+ cores. A pass over arXMLiv 08.2018 now takes between 2-3 hours for a lightweight task (frequency reports, token models, etc) on such hardware.

The library also uses the parallel-friendly RoNode libxml struct, which allows for additional gains when iterating over the DOM.

Example from corpus_mathml_stats:

use llamapun::parallel_data::Corpus;
// ...
let corpus = Corpus::new(corpus_path);
let catalog = corpus.catalog_with_parallel_walk(|document| {
  document
  .get_math_nodes()
  .into_par_iter()
  .map(|math| {
    let mut catalog = HashMap::new();
    dfs_record(math, &open_ended, &mut catalog);
    catalog
  })
  .reduce(HashMap::new, |mut map1, map2| {
    for (k, v) in map2 {
      let entry = map1.entry(k).or_insert(0);
      *entry += v;
    }
    map1
  })
});

For details, consult #29