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.
And update tests as well.
The content-flags won't change, and the unread-flag can be re-calculated. Add a unit test, and some small doc improvements. Fixes #2831.
Add sub-command to move messages; add tests and docs. Fixes #157