Boost PFR as implicit fallback

[denzor200]

#231

This PR suggests to use PFR in the Fusion like implicit fallback. It means that Fusion will try using PFR for types that isn't a Fusion Sequence.
This is an optional functional enables by inclusion of special header (adapted/boost_pfr.hpp, for example), or it enables by inclusion of just adapted.hpp. If you don't need this, just don't include special header, or disable PFR using special macro( BOOST_PFR_ENABLED, for example ).

If you need to use PFR, but you don't want to use it like implicit fallback, you may disable implicit reflection and use explicit instead. Explicit reflection requires from user to specify all reflectable types manually using one of the suggested macroes.
Explicit reflection is provided by the header adapted/boost_pfr_explicit.hpp.

It's able to play with this PR here: https://godbolt.org/z/vMMf8GTE4

[denzor200]

@djowel, @daminetreg , @Kojoley I'm ready to show this PR.
Would you like to join the review?

[denzor200]

@djowel Hello, are we going on?

[daminetreg]

Wow awesome that's terrific. I missed your comment from 29th January. I will do my best to review and try it out this Friday.

[denzor200]

@daminetreg Cool!

[denzor200]

@daminetreg Hi! I want to inform that I may be unavailable a month later for some time. Can we do something with this PR right now, or we will just postpone it for some time?

How is your review going on, is there any trouble or just not enough time for it? Can I help you with something?

Maybe I should reduce this PR throwing away the implicit reflection? We would merge the explicit one pretty fast, and then leisurely continue working on huge implicit reflection having already released the first PFR integration.

[daminetreg]

Hi @denzor200 , sorry for the delay I couldn't try yet but I will do my best to give my early feedback this week still. I think it's a very awesome feature, so we should not wait too long for merging. I apologies for my bandwidth here.

[xR3b0rn]

Any updates on this?

[denzor200]

@xR3b0rn everything ready from my side, waiting for review. As I informed above, I was unavailable for some months. But right now I'm available every day and can react to review pretty fast.

@daminetreg