Split core solver from the theory of strings (#3713)

Split core solver from the theory of strings (#3713)

This splits the main procedure from Liang et al CAV 2014 to its own file, the "core solver" of theory of strings.

I have intentionally not updated or clang-formatted the code in core_solver.cpp since I would prefer this PR to involve as little change to behavior as possible (it is copied verbatim from theory_strings.cpp). Future PRs will clean this code up.