Setting `__dict__` breaks PEP 412

[psippola]

In pydantic v1, replacing each instantiated object's __dict__ with a dict of validated values used to broke
PEP 412 (use of shared dictionary keys), which probably explains a part of the excessive memory usage of V1 when a lot of objects of same type were created.

This may be a concern still in v2, if I read rows like this correctly:

force_setattr(py, model, intern!(py, DUNDER_DICT), validated_dict)?;

I have no adequate rust skills to test this, but it could be beneficial to study how shared keys affect memory consumption of objects validated by pydantic, and could updating __dict__ instead of replacing result in a smaller memory footprint.

More about shared keys, I hope that this is still relevant:

Internals of sets and dicts