Enum QoL Features

[Blacksmoke16]

• Add an argument resolver that handles resolving enum members based on the request value
  • Handles both numeric and string based conversion
• Introduce ART::Requirement::Enum to make defining route requirements easier for enums
• (breaking) Replace ASPEC::Methods.assert_error to leverage non-file version
  • Update ATH and ADI compiler specs to use it
• Remove is_nilable property from ArgumentMetadata
  • This can always be inferred from T
• Add methods to ArgumentMetadata to allow determining if it is an "instance of" another type as well as returning the first type that matches the provided metaclass
• (breaking) Return underscored strings by default when serializing enums
  • Is inline with Serialize Enum to underscored String by default crystal-lang/crystal#10431