Add parent tracking to visitor

[s-coimbra21]

Add parent tracking using a stack to track the path from the current node being visited to the tree root. I'm using a ParentNode enum with two variants, one for ExpressionNode and another for StatementNode which covers all possible visitable nodes that may have children, the variants store a reference to the actual node so I had to change the lifetimes to &'ast explicitly on the Visitable trait, perhaps there is a better way.

[maciejhirsz]

Will review in depth when I can, but just glossing over the code, it looks really good!