Add helper functions for creating Let expressions. Add simple type checking test for Let expressions.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Cache results of the normalizer. Add example that demonstrates the exponential performance improvement.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Fix mpfp::operator^= to resolve operator overloading problem
Fix precision problem in interval<T>::cos
Add more tests for float and double interval