is_compatible_* traits were used in from_json, but it made no sense
whatsoever.
It used to work because of non-SFINAE correctness + json_ref
unconstrained variadic template constructor.
SFINAE checks are becoming quite complex, we need a specification of
some sort describing:
* which concepts the library uses
* how the conversion to/from json works in detail
Having such a specification would really help simplifying the current
code (as well as having meaningful checks).
Fixes!1299
* Make the conversion operator SFINAE correct.
* Workaround a GCC bug with some traits in type_traits.hpp
The first bullet-point implies that every `get`/`get_ptr` be SFINAE
correct as well.
