Make ExtTheory independent of Theory (#5010)

Make ExtTheory independent of Theory (#5010)

This makes it so that ExtTheory uses a generic callback instead of relying on Theory.

The primary purpose of this commit is to eliminate the connection of TheoryBV and ExtTheory. This commit moves all things related to ExtTheory in BV into CoreSolver.

It also refactors the use of ExtTheory in strings and arithmetic.