Improvements and tests for the API around separation logic (#2229)

Improvements and tests for the API around separation logic (#2229)

- Introduces a system that validating that, when not using THEORY_SEP, that it is not possible to
  obtain the separation logic heap/nil (validate_exception())
- Introduces a system test demonstrating how to use the separation logic theory, and then how
  to use the "getters" to obtain and interrogate the heap/nil expressions (validate_getters())
- Refactors the original getters to avoid duplicate code
- Add a check as part of the getters to ensure that THEORY_SEP is in use