Simplify cast int exprs and compares w extremes

Simplify expressions involving cast integers, computing and storing the
canonical representation of the result.
Simplify comparisons, max and max involving extreme values of integers.
Simplify: fixes; use new mod and div; test

Fix simplify handling of unsigned cast integers.
Fix minor bug in simplify And, Or: not using mutated args.
Add test of simplifier to div_mod.
Merge pull request #69 from lenhamey/simplify-2

Simplify cast int exprs and compares w extremes