Revisiting Slots usage on key data abstractions

A good portion of our classes define their fields in __slots__. We also use slots for JSON Serialization (see the JSON module). The slots can be frustrating because they prevent any custom dictionary extensions without subclassing the relevant dictionary classes.

We should considering doing one of two things:

• Figure out an alternative way of dealing with serialization and the reliance on slots
• Live with __slots__ and the like but document clearly the necessity of slots and its uses in the extension documentation so there's no surprises for users.

Start with ait/core/json.py and inspect the project’s uses of slots in key data abstractions. Determine whether the issue should pursue an alternative serialization approach or document the existing constraint in the extension documentation; done requires a clearly chosen direction and corresponding coverage.