Eliminate remaining references to parent TheoryStrings object (#4315)

Eliminate remaining references to parent TheoryStrings object (#4315)

This PR makes it so that the module dependencies in the theory of strings are acyclic. This is important for further organization towards proofs for strings.

The main change in this PR is to ensure that InferenceManager has a pointer to ExtTheory, which is needed for several of its methods. This required changing several solvers into unique_ptr to properly initialize.