Integrate boost::pfr into hana

[denzor200]

https://github.com/boostorg/pfr

It's a small library that provides basic reflection for user defined types. Now the library have everything required from Hana's Struct concept(including reflection of field names) and might be integrated in order to get rid of BOOST_HANA_ADAPT_STRUCT macro.

Don't hesitate to ask any question regarding PFR, my pleasure to provide as much help with development as it required.