Reassign state Var when fields on a Base instance change

[masenf]

Refactor mutable type handling in State classes:

• Create new MutableProxy object that wraps list, dict, set, and Base-derived objects coming from the State.
• When certain __magic_attrs__ are accessed on wrapped objects, and they are callable, mark the associated var field in the state as dirty.
• When accessing attributes or items through the proxy, and the resulting object is also mutable, then also wrap it in MutableProxy associated with the original state and field_name.
• When assigning a MutableProxy back to the state, actually assign the wrapped object.

This approach does not incur as much runtime overhead as the current implementation, since it does not create copies of objects just for state tracking and does not create copies of objects when assigning back to the state. Instead it just wraps the original mutable object.

Fixes #1675

[masenf]

Yikes, this appears to approximately double the time taken by tests/test_state.py 😱

[masenf]

So I was thinking about this one a bit more... and instead of doing a full re-assignment (and reinitialization, etc) for these mutable types, we probably should just mark them dirty in the state.

Essentially, we will add some code to State.__getattr__ which returns an object proxy for mutable types, when any attribute is set or mutation method called, the proxy will mark the field as dirty in the state, and then pass the call through to the underlying object.

Then State.__setattr__ will be extended to identify when attempting to assign an object proxy and just assign the object itself instead.

With this approach, we won't need specialty collection objects and we won't need slow up-front depth first traversals. We can just mark the fields appropriately at runtime when they are accessed/modified.

[picklelo]

nice, this solution ended up very clean :)