only check for public types (iterators, json_pointer)

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

removes:
- is_basic_json_compatible_type traits
- a useless operator=
- void_t
- disjunction

changes:
- is_compatible_object_type's first argument is BasicJson

- 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