std::hash extensions

hash_extension.cpp

template<typename T, typename U>
struct std::hash<std::pair<T, U>> {
    std::size_t operator()(std::pair<T, U> const& value) const {
        std::size_t thash = std::hash<T>{}(value.first);
        std::size_t uhash = std::hash<U>{}(value.second);
        // See boost::hash_combine for this
        return uhash ^ ( thash << 1 );
    }
};