CountMinSketch::HashFunction returns different results on Ubuntu and macOS

[max-kamps]

On Ubuntu, clang uses libstdc++ (GNU) by default, while on macOS it uses libc++ (LLVM) by default.

CountMinSketch::HashFunction invokes std::hash (in particular, std::hash<std::string> during testing), which has different implementations in libstdc++ (MurmurHashUnaligned2) and libc++ (CityHash).

I think this is probably a mistake? CountMinSketch::HashFunction should not need to use std::hash, since there is already a stable hash function in hash_util.h