Allocation optimisations & Clippy cleanup

[michaelsproul]

Proposed Changes

I started trying to get the Clippy lints to pass and then I got a bit carried away. I ended up removing all indexing (which could panic) and unwraps. And I found a way to get rid of some intermediate collects by messing with the validity filters (making them safe to share between threads).

I also reversed the polarity of the hash_set_filter predicate, so that it matches Iterator::filter.

[GeemoCandama]

I always forget about clippy until the end sorry. Making validity_filter Fn and Sync makes the get_cliques function so much nicer. The AtomicUsize is a new one for me so I'll have to remember that.