[FIRRTL] Add folders for IntegerMulOp

This is a primitive Property operation, defined by FIRRTL spec section 25.1.2. 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_mul(a, 1) -> a
* [ ] integer_mul(constantA, constantB) -> constantA * constantB
* [ ] etc...

Start by locating the FIRRTL IntegerMulOp implementation and read FIRRTL specification section 25.1.2. Review existing constant-folding patterns for primitive Property operations, then implement the listed multiplication folds, including multiplication by one and two constants. Done means the supported folds are covered by tests and the examples produce the expected results.

Written by the indexing model from the issue text.