Introduce reportChanged API to keep state about whether a visitor changed the AST:

* reportChanged is both methods on the context and the visitor.
* the user calls the context method and the context method calls the visitor method.
* This is optional so the API names tries to reflect this: e.g. `wasChangeReported()` vs `isChanged()`
Merge pull request #79 from amasad/master

Introduce reportChanged API to keep state about whether a visitor changed the AST.