C++23

[mcandre]

• [ ] size_t literal syntax https://en.cppreference.com/w/cpp/types/size_t
• [ ] #elifdef, #elifndef https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2334r1.pdf
• [ ] contains() member for strings
• [ ] map, and_then, and or_else members for optionals
• [ ] std::unreachable

Pending #323

Pending wide support for C++23 features in a version of clang included in the stable releases of Debian and Ubuntu and Raspbian and macOS (Apple Xcode clang, don't care about Homebrew's clang).