Consider adding optional template argument for custom "map" implementation and keep the default

[Alexander-Shukaev]

As per subject.

[jimporter]

Indeed. When most of this code was written, std::map was the only (standard) associative container with heterogeneous lookup, but as of C++20, std::unordered_map allows this too. (And there are the std::pmr types in C++17, though I'm not sure they'd be especially useful here.) And, of course, it'd be nice to be able to use some kind of LRU cache.