More minor optimizations for datatypes.
Add option --dt-stc-ind for strengthening skolemization.  Refactor skolemization.
Fix assertion from previous commit.
Simplification of EqualityEngine::areDisequal.  Comparison for production : http://church.cims.nyu.edu/regress-results/compare_jobs.php?job_id=6444&reference_id=6445&p=5.  Comparison for debug : http://church.cims.nyu.edu/regress-results/compare_jobs.php?job_id=6449&reference_id=6446&p=5.