Integrate clang-format

[nirs]

To keep consistent format without effort, we want to have automatic code formatting. clang-format seems to to be the best tool for this.

Taks:

• Document clang-format developer requirement in CONTRIBUTING.md
• Add CI auto-format check, failing if code is not formatted
• Pick one of the builtin styles (LLVM, GNU, Google, Chromium, Microsoft, Mozilla, WebKit)
• or add a .clang-format file to match our style
• Find how to keep aligned text - seems that we need to skip the formatter around aligned text
  • seems to be `// clang-format on|off comments: Improve formatting #101 (comment)
• Change the style to something more readable? 2 spaces are not great

Links:

• https://clang.llvm.org/docs/ClangFormat.html#clangformat
• https://www.kernel.org/doc/html/next/dev-tools/clang-format.html