Upgrade to cpp20

[kfu02]

we should make all of these is_done methods also constexpr. You cannot make virtual functions constexpr in cpp17 which we use right now, but when I switched to cpp20, this change worked. Marking as constexpr offloads all the work to compile time. Since doing this requires changing our cpp version, we can do it in a different pr if you want. I still think it should be done at some point.

Originally posted by @kasohrab in #1911 (comment)