Simplify input in preprocessor

[rtaupe]

State-of-the-art ASP systems employ sophisticated preprocessing to simplify input programs, thereby improving solving performance. Probably Alpha could benefit from such techniques as well.

Example reference: Gebser, M., Kaufmann, B., Neumann, A., & Schaub, T. (2008, June). Advanced Preprocessing for Answer Set Solving. In ECAI (Vol. 178, pp. 15-19).