Neighborhood trait + Unsortedneighborhood impl

[mcalancea]

• adds the Neighborhood trait which captures the HNSW search requirements of the candidate list data structure.
• adds the UnsortedNeighborhood as an implementer which produces candidates in arbitrary order and uses quick-select to periodically trim its size.
• rewrites methods in HnswSearcher to be generic in N: Neighborhood.
• adds dynamic dispatch of N (for its two concrete impls, Sorted and Unsorted) at certain entry-points (search::search for hawk_main and run_accuracy_analysis.rs). adding more entry-points is doable with minimal effort.
• simplifies some existing data structures