feature: CtScanner-based DFS Iterator for a CtElement

[msramalho]

I am working with Spoon for a project and needed an iterator that would go over a CtElement's descendants in a depth-first search manner, since CtScanner does not do this one element at a time, but rather at once (using enter() and exit()) it was not fitted for my designs. So I created a CtIterator.

If you can't see why this would be needed instead of an extended CtScanner consider you have two elements and need to go through both their trees but only advance on one, when a condition is met on the other and vice-versa.

Example behaviour

CtIterator iterator = new CtIterator(/** some CtElement*/); 
while (iterator.hasNext()) { 
    CtElement next = (CtElement) iterator.next(); 
    System.out.println(next.toString()); 
}

If you would like me to create a PR to spoon with my implementation of CtIterator, please say so. Otherwise just close this issue.

😃

[monperrus]

Welcome to Spoon Miguel! This sounds really interesting. We'd be happy to review and merge your PR :-)