The rewrite uses more cmake build-in automatisms and build-in generates
variables to allow better generic reuse.
* cmake files are installed to
``` <install_prefix>/lib/cmake/nlohmann_json/ ``` for best support on
most systems
* include path is set to ``` include ``` for usage as ``` #include
<nlohmann/json.hpp> ```
- 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
- define the project's version in the cmake listfile
- create a config and config version file for the cmake package
- install the json.hpp file and the package's targets, config and
config version files
