RFC: clang-format all the things!

Re. tool versioning: It appears that the CI has to use a released version, currently set to 17.0.1. I haven’t been using that version for my experiments but precise numbers aren’t the point, it’s the scale of the thing that matters right now, and I’m confident that the more-or-less-HEAD version I’m using isn’t different enough to make that data trash.

When it comes time to start updating files in the repo, then the specific version will matter.

If we do this in stages, early stages might discover things that need to be addressed in the tool. If we do that efficiently, we can still get fixes into 18.1.x and then pin the CI to that version going forward. Yes, there’s a chance this will mean reformatting some number of files, but I think we can live with that number being kinda small, and not require it to be zero.