Support ACORN-1 by visiting 2-hop-neighbors.

This is the first time to modify the C++ usearch code. This is just the proposal of ACORN-1. Please consider this change and amend whatever you feel appropriate.

• So, to implement ACORN-1, when visiting a node in the HNSW search, we take the neighbors and 2-hop neighbors of the node, and for those that pass the filter, computes their distance to the query vector to see if they are relevant.
• stick to original HNSW model and ACORN-1-inspired heuristic only on the lowest layer of the graph.