Properly constrain the basic_json conversion operator

* 🔥 clean CMake files and Makefile
* 💡 update documentation

* ♻ split Unicode tests

Fixes #2491

Avoid string in case of empty CBOR objects

Add a cmake option to use SYSTEM in target_include_directories

Suppress C4127 warning in unit-json_pointer.cpp

Fix truncation warning

Fix memory leak in to_json

 Conflicts:
	include/nlohmann/detail/output/serializer.hpp
	single_include/nlohmann/json.hpp

Specified git branch for google benchmark fetch in benchmark test