zhashmap: Fast open addressing hash table in C++

[JeGX]

This hashmap is an open-addressing hashtable similar to google/dense_hash_map, however, it uses tombstone bitmaps to eliminate necessity for empty or deleted key sentinels. This means the hash map could potentially be made into a drop-in replacement for std::map or std::unordered_map. The code supports enough of the C++ unordered associative container requirements such that it can be substituted for many typical use-cases.

These small maps are useful for histograms and other frequently accessed data structures where performance is critical.

Link:
- https://github.com/michaeljclark/zhashmap