Concurrent hash table and map, and some improvements. ### New Features - Cache of hash numbers ([Issue #11](https://gitlab.com/MusicScience37/cpp-hash-tables/-/issues/11)). - Concurrent hash table and map ([Issue #8](https://gitlab.com/MusicScience37/cpp-hash-tables/-/issues/8), [Issue #10](https://gitlab.com/MusicScience37/cpp-hash-tables/-/issues/10)). ### Improvements - Refactoring of `open_address_table_st` according the results of profiling ([Issue #4](https://gitlab.com/MusicScience37/cpp-hash-tables/-/issues/4)). ### Others - Further benchmarks ([Issue #7](https://gitlab.com/MusicScience37/cpp-hash-tables/-/issues/7)) - Updated documentation ([Issue #9](https://gitlab.com/MusicScience37/cpp-hash-tables/-/issues/9)).