`VisitorPartialEvaluator` should group constant terms when possible like `((a + 2) + b) + 3 = (a + b) + 5`

Some things to consider:
- might not want to do this when function calls are involved, because they could have side-effects. On the other hand if the order of the calls do not change, it should not make a difference? For example: `doA() + 2 + doB() + 3` should not be turned into `doB() + doA() + 5`