Apischema

- Fix default_schema not being called

- Improve Optional handling in schema generation
- Improve deserialization performance for dataclass primitive fields
- Add ValidationError deserializer
- settings.default_schema
- Pydantic compatibility example

- Fix openAPI definitions path
- Add default ref for other standard types or types with schema

- Update package description

- Fix support of `Generic` merged dataclasses

- Add `InitVar` support for merged dataclasses