[FIRRTL] Add folders for IntegerAddOp

[mikeurbach]

This is a primitive Property operation, defined by FIRRTL spec section 25.1.1. General folds for signed arbitrary precision integer arithmetic can be applied, but none were defined in the initial PR. This could include things like:

• [ ] integer_add(a, 0) -> a
• [ ] integer_add(constantA, constantB) -> constantA + constantB
• [ ] etc...