improve static data structures

[barbieri]

Some static data structures, such as resolver map and the validation information could use some better data structures, in particular avoiding runtime computation, trading it for compile-time whenever possible.

https://blog.quarkslab.com/frozen-an-header-only-constexpr-alternative-to-gperf-for-c14-users.html

could be interesting to this project and the generated code.

[wravery]

I realized that the internal::sorted_map and internal::sorted_set should already be convertible to constexpr in C++20. The new standard enabled constexpr versions of std::less, std::sort, std::vector, and std::equal_range. All it should take is declaring the internal::shorter_or_less comparator constexpr and declaring the constructors and other methods on these types as constexpr as well.