- Add prefix header
  - Include catch.hpp
  - Include json.hpp
    - Replace private with public for all json_unit files
- Move `unit.cpp` to an object library
- cotire issue: strip whitespace from CMAKE_INCLUDE_SYSTEM_FLAG_CXX

Overload comparison operators for all types that could be converted
to nullptr.

only check for public types (iterators, json_pointer)

for private ones (m_array.iterator, primitive_iterator) simply double
parentheses on relevant Catch checks

- tweaked a bit how `get<container<json>>` is handled
- added a from_json overload for forward list

this also means that one can do: j.get<unscoped_enum>();

This commit forces a design change in custom JSONSerializer, which might
be temporary