Api versioning

This PR solves the issue https://github.com/msgpack/msgpack-c/issues/135
msgpack-c can provide multiple versions of APIs. Users can write custom overload functions.
Users need to write a custom overload as https://gist.github.com/redboltz/3ef4718a9e4429b21e21
Because msgpack-c's operator<< and >> no longer depends on ADL.