Using rayon alongside mutex can cause random deadlocks

[rhysnewell]

Bug 🐛

Rayon does not handle mutex like I thought it did. Occassionally results in the program deadlocking when running through a parallel iterator if that iterator is altering a mutex: rayon-rs/rayon#592

Fix 🛠️

Refer to rayon documentation here: https://docs.rs/rayon/1.3.1/rayon/iter/trait.ParallelIterator.html#method.for_each_with
Instead of using Mutex, use channel() from std::sync like so:

        // Create condensed links sender and receiver, avoiding use of mutex
        let (condensed_links_s, condensed_links_r) = channel();

        // extend the links for each anchor point by the union of all the indices
        links.par_iter().for_each_with(condensed_links_s, |s, (_, current_links)|{
            let mut anchors = current_links.clone();
            current_links.iter().for_each(|index| {
                match links.get(&index) {
                    Some(other_links) => {
                        anchors.par_extend(other_links.par_iter())
                    },
                    _ => {},
                }
            });
            s.send(anchors).unwrap();
        });

        // Collect receiver into hashset
        let condensed_links: HashSet<_> = condensed_links_r.iter().collect();

Issues 😢

Any HashMap types that are Arc<Mutex<_>> will be difficult to send through channel(). Not sure how to do that yet.