Sources for these 3rd party hashmap implementations:

* google: https://github.com/sparsehash/sparsehash
* rigtorp: https://github.com/rigtorp/HashMap
* sparsepp: https://github.com/greg7mdp/sparsepp
* tessil: https://github.com/Tessil/hopscotch-map
* sherwood_map: https://probablydance.com/2014/05/03/i-wrote-a-fast-hash-table/