* Make @rx.memo work with state vars passed as props
Seems like this was a regression from the StatefulComponent refactor, because
trying to pass a state Var to a CustomComponent gave undefined, likely due to
`_get_vars` not accounting for `self.props` in CustomComponents.
With this change, it works.
Integration test added to `test_var_operations.py`
* Allow CustomComponent props to be Component
Avoid calling `.json()` on all Base types because the Var serializer already
does that, but this way, more specific types (like Component) can be serialized
differently.
When the type is Component, attach a VarData with the imports and hooks to when
the Var is rendered, it also carries the correct imports/hooks and does not
throw frontend errors.
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