Postpone conversions

[ggabriel96]

General description

This is a step back from what we currently have so we can:

1. avoid using std::any
2. only ask the user once for the desired type
3. prepare for The return of templated Arg? #5
4. prepare to support different actions

Technical details

• changes in types:
  • Arg now only has name, help, and required and becomes an aggregate
  • removed ArgInfo
  • ArgValue stores a std::string instead of std::any
  • gave up on operator T (see discussion)
  • new member functions get_if_else, value_or, and as in ArgMap
  • SplitArg becomes ParsedOption
• adapted converters, replacing std::optional<std::string> with std::string
• changes in Program:
  • member function add break into pos, opt, and flag (currently lacking needed checks)
  • removed add_choice_checking_to_conversion
  • directly storing Arg
  • separate containers for flags, options, and positional arguments
  • with separate containers, is_flag only needs to check if the flags container has the given name
  • convert_args becomes assign_args
  • only assign parsed arguments (default values will come later)
  • now handling too many dashes
  • now detecting user-provided values for flags during parsing
• other changes in src/opzioni.cpp:
  • refactored split_arg into parse_option
  • count_dashes replaces is_two_dashes
• exception ArgumentNotFound was not being used, so got replaced by MissingRequiredArgument; new exception TooManyDashes
• temporarily removed tests in src/types.test.cpp
• commited variant.cpp by mistake, but now it's saved for later 🤷‍♂️ (will use it as a base for the std::variant implementation mentioned earlier)