Gata

- Removed dataclass usages from internal implementation
- bugfixes

- Fixed problem with empty doc parsing

- Fixed `gata.transform` method signature

- added `gata.transform(obj, transform_to, mapping=None)`
- refactored schema codebase

- `Any` type now is ignored during serialisation and deserialisation
- `serialisable` and `validatable` decorator does not require `dataclass` decorator anymore