Split term registry from theory state in sets (#5037)

Split term registry from theory state in sets (#5037)

Currently, the theory state object SolverState in sets sends lemmas and has a reference to the parent theory. This PR is work towards eliminating these dependencies.

This adds a TermRegistry object which is responsible for some of the tasks currently done by SolverState, including all those involving lemmas.

Notice this also makes a bug fix in getUnivSetEqClass where the universe set was being returned instead of its representative.

A followup PR will make SolverState maintain SAT-context-dependent membership lists which is also required for breaking the dependence on the parent theory.