fixed rewriter bug where postRewrite was not caching properly
Support for parametric datatype subtyping, so that e.g. (Pair Int Int) is a subtype of (Pair Real Real).  Resolves bug #541.
Add test case for (previously resolved) bug 528.
Minor cleanup.
SExpr pretty-printing for :all-options and :all-statistics.
Better approach to forcing string logics to contain UFLIA also.