Paper to follow:
https://github.com/steve-downey/unicode-formal/blob/main/src/unicode_formal/unicode_formal.h

Based on the ICU C macros, but once types are added, some of them are not
very useful, or plain harmful.

These are all classifications and counts, no decode or encode, or validity
checks above code unit.

I haven't added the various named block functions, as I don't really see a
particular point to them outside of short-circuit real unicode database
functions.

They're all constexpr inline noexcept. Some may have 'unspecified' results
where that means the value is deterministic but meaningless. There is no
undefined behavior.