Add hash_value overloads to also support boost::hash in addition to std::hash

[pdimov]

Now that https://github.com/boostorg/json/pull/522 has added std::hash support for the JSON types, adding hash_value overloads that simply return the result of std::hash (or vice versa) will also enable support for boost::hash.

Apart from the fact that a Boost library should integrate with other Boost libraries, boost::hash is also more convenient in that it supports the standard containers, std::pair and std::tuple.