Add tree transformation capabilities to Node and Tree

[danieldietrich]

Wire together tree traversal (with walkers: preorder, inorder, postorder, level-order) and tree transformation.

Suggestion: Take the rewrite-rule approach using the Match API (enhanced by #8).

[danieldietrich]

Useful links:

• Stratego rewrite rule
• treehugger.js
• treehugger.js playground
• wikipedia: tree traversal
• wikipedia: tree data structure

[danieldietrich]

I want Tree transformation look like this (stolen from databricks):

tree.transform {
  case Add(Literal(c1), Literal(c2)) => Literal(c1+c2)
  case Add(left, Literal(0)) => left
  case Add(Literal(0), right) => right
}

[danieldietrich]

We need a function

Tree.transform(Function<? super Tree<? super T>, ? extends Tree<? extends T> f)

or

Tree.transform(UnaryOperator<Tree<T>> f)

Currently a Node has children of type List<Node>. We need to allow the empty tree and change the type of Node's children to List<Tree>.

[danieldietrich]

Note: Removed the transform method because tree.transform(f) is the same as f.apply(tree).