K-format

- Fix `N` type to handle negative numbers properly (7)

- Fix value overwriting bugs when creating multiple K-Class instances (4bae74dd8d73aa07c62e753db6e9b8b14c9592bd)

- Improve `K-Class` to accept other `K-Class` type property (4)
- Improve `K-Class` to accept list of `K-Class` type(`List[KClass]`) property (5)

- Fix `AN` property type to accept `None` (2)
- Improve type assertion error message when constructing K-Class (3)

- You can create your own K-Format class(`kclass`) just like dataclass