CI should check code with clang-format and/or clang-tidy

[Rangi42]

We used to use checkpatch.pl, but it's for C, not C++, and doesn't follow the style conventions we want.

clang-tidy src/**/*.cpp -- -Iinclude -extra-arg=-std=c++20 gives lots of false positives (I might be using it wrong), but might be a good starting point for CI or at least a manual analysis.

Most files will need clang-formatting, plus manual work to get rid of old conventions like "blank line after declaration" and "no braces for single-statement control blocks".