- removed ParseResult (parsing directly into ArgMap)
- list of supported types in BuiltinTypes
- Arg has std::variant default value member
- Arg stores its action
- implemented very basic assign and append actions
- removed Program::assign_* member functions
- temporarily not assigning default values when setting parsed arguments
- just updated unit tests to pass, gotta re-do them

- moved Program to its own .hpp/.cpp files
- types.hpp/.cpp renamed to args.hpp/.cpp, added "section comments"
- opzioni::parsing::parse_option moved to program.hpp, not in parsing namespace anymore
- include cleanup
- removed unused opzioni.cpp and types.test.cpp