[feature] Immutable, hashable models, with text representations

[lmmarsano]

Similar to objects we get from frozen dataclasses or frozen pydantic models, simple data that we can compare and treat as values would be useful for debugging (eg, stack traces in test frameworks that reveal values) & general productivity. Basically, define

• __hash__
• __repr__

and block setting properties. Models with different attributes can already be created via serialize & deserialize methods.