Use code formatter/linter for C++ code

[CargoCultism]

Try clang-format:

• [x] Apply manually and document how
• [ ] Check if writing script is a good idea, if yes write
• [ ] Add CMake target for clang-format

[CargoCultism]

Open question: If clang-tidy target specified via CMake, how to beautify original source files? ${CMAKE_SOURCE_DIR}?