Simplify management of separation logic heap (#8580)

Simplify management of separation logic heap (#8580)

The separation logic heap is conceptually an extension of the logic, which lives in Env. This PR moves the separation logic heap types from TheoryEngine to Env.

It furthermore makes it so that declaring the separation logic heap does not force initialization of the SolverEngine, meaning that further declarations/options may be done after setting the heap.

This is in preparation for making the SmtSolver class easier to deep reset.