Create a SpoonUtils utility class with reusable Spoon transformations

[slarse]

The method not(CtExpression<T> expr) is a copy from another processor. We should have something like SpoonUtils to keep these utility methods together and accessible to all processors. I believe this should be handled in a different PR though.

Originally posted by @fermadeiral in #571 (comment)

To summarize that comment, some Spoon transformations are reused across different processors. This is a concrete example, but I'm sure we can find other transformations that are duplicated (either as parts of methods or methods of their own).