HNSW support

Just curious if the HNSW algorithm is amenable to use in this project? Probably not directly because I see you are jitting in the distance metric but curious if it is worth a look into that algorithm for the nn search vs random projections? It is consistently the top performing algorithm on the ann shootout and I was wondering if the performance of UMAP is significantly impacted by the NN component and if so would incorporating HNSW make a meaningful difference?