struct

1. Create simple struct
2. Change attributes of a mutable struct
3. Struct method to modify fields
4. Struct inheritance
5. Struct composition
6. Struct duplicate
7. Printing struct fails
8. Print struct (Point)
9. Debug struct (Point)
10. Print complex struct
11. Debug complex struct
12. Struct with vector and optional value
13. Printing and debug-printing simple struct
14. Use a tuple as a struct to represent color
15. Add method to tuple-based struct
16. Struct with method
17. Structs and circural references
18. Multiple referene to a struct
19. new method with default values for struct
20. The new method has no special feature
21. Default values
22. Default for composite struct
23. Compare structs for Equality
24. Compare structs for Equality - manual implementation
25. Compare structs for partial equality - PartialEq
26. Compare structs for ordering (sorting) - Ord
27. Compare structs for partial ordering (sorting) - PartialOrd
28. Manually implement ordering