With the previous Walk function it's not easy to rewrite the node as we
don't have any kind of reference to the parent. If we want to rewrite a
given AST, we have to manually traverse it as Walk is not usable. To
allow us rewriting the AST we change the signature of the function
passed to Walk. It'll allow us to rewrite the AST and return back.
Internally Walk() overrides the returned AST.
This idea was also talked here:
https://groups.google.com/forum/#!topic/golang-nuts/cRZQV36IckM
extensively.
