Move the various seq_ functions, as well as std::(stable_)sort,
std::accumulate, std::transform, std::find, std::find_if to their C++20
std::ranges counterparts.
Update all cc code using .clang-format; please do so as well for future PRs
etc.; emacs has a handy 'clang-format' mode to make this automatic.
For comparing old changes with git blame, we can disregard this one using
--ignore-rev
(see https://www.moxio.com/blog/43/ignoring-bulk-change-commits-with-git-blame )
