Better fix for bug 528
Reduce compiler dependencies on substitutions.h,
Some new functionality in substitutions.h/cpp
adds model generation for strings, and a hacked way in arith engine for models
optimizing model generation for strings
fix loop detection for multi-vars
for morgan to see the regression problems
fix the infinite issue
removes unsound cases, adds unrolling
Merge branch 'master' of github.com:tiliang/CVC4