Validating existing models

[samuelcolvin]

As per #21 we need to make sure values are copied.

As @tiangolo points out at in pydantic/pydantic#4218 (comment) we need to be able to validate a model without relying on isinstance.

I think we should therefore change how existing models are validated to effectively revalidate model.__dict__. That should solving copying (of models at least) and avoid subclasses being validated as parent classes.

This might have some performance impact, but it'll be much smaller than a hack in python to work around it.