Integrate Boost.Describe into Boost.Hana?

Briefly about **the Boost Describe library**:
"The purpose of the library is to establish a standard way of providing these reflection abilities. Many existing libraries provide their own way of describing enums or classes, but without a standard, code written by different people cannot interoperate." © Peter Dimov https://www.boost.org/doc/libs/develop/libs/describe/doc/html/describe.html

For example, this will allow _BOOST_DESCRIBE_STRUCT_ to be used instead of _BOOST_HANA_ADAPT_STRUCT_, which is more standarted way.
I suggest to add _boost/hana/ext/boost/describe.hpp_, which will take over the integration with this library.

Start with the Boost.Describe documentation linked in the issue and inspect the existing Boost.Hana extension layout around boost/hana/ext/boost/. Determine how BOOST_DESCRIBE_STRUCT should relate to BOOST_HANA_ADAPT_STRUCT, then treat a completed boost/hana/ext/boost/describe.hpp integration and appropriate coverage as done.

Written by the indexing model from the issue text.