I think we should patch test/CMakeLists.txt to always use -Wall -Werror on *nix and the corresponding MSVC flags. It is way too easy to overlook a warning in the CI output as #1797 has shown.

@nlohmann Thoughts?

@cyrillos says xxhash is much better than Murmur which we use now. Up to 2 times in speed from what he showed in some benchmarks. It must be investigated, because we use murmur for some perf critical places.