Provide a substitute to BOOST_FUSION_ADAPT_STRUCT_xxx macros

Would it be possible to provide a substitute for `BOOST_FUSION_ADAPT_STRUCT_xxx` macros to be able to use classes and structs as heterogeneous (+associative?) sequences?

I don't know if there are any C++>=11 features that would allow improving these macros or designing a new/better version of them, but that might be worth looking into.

Start by reviewing the existing BOOST_FUSION_ADAPT_STRUCT_xxx macros and how they represent classes and structs as heterogeneous sequences. Define what a replacement must support, including whether associative sequences are required, then verify that the proposed design works with C++11 or newer constraints.

Written by the indexing model from the issue text.