Unify top-level substitutions and model substitutions (#6499)

Unify top-level substitutions and model substitutions (#6499)

This unifies top-level substitutions and theory model substitutions. Env is now passed to the TheoryModel, which has access to top-level substitutions.

The former was used for simplfying assertions in the preprocessor, the latter was used for evaluating terms in the model.

There is no reason to have these two things be separate.