Classes

1. Empty Class definition
2. Class with attributes
3. Class with getters
4. Class with setter
5. Class with getters and setter (property)
6. Class with property with default value
7. Class with declared getter and default value
8. Class with declared getter
9. Serialize Crystal-lang class to/from JSON from_json to_json
10. Compare objects for equality
11. Singleton using class properties
12. Singleton using class properties with default values
13. Class monkey-path add method
14. Stringification to_s