Make sets and strings solver states inherit from TheoryState (#4918)

Make sets and strings solver states inherit from TheoryState (#4918)

This is towards the new standard for theory solvers.

This PR makes the custom states of sets and strings inherit from the standard base class TheoryState. It also makes a minor change to InferenceManager/SolverState to make sets more in line with the plan for a standard base class InferenceManager.

Followup PRs will establish the official TheoryState classes for all other theories (which in most cases will be an instance of the base class).