Use estraverse in tree-iterator.

I suggest removing `tree-iterator` logic and use `estraverse` instead:

* It takes time to support `tree-iterator`.
* It will take even more time to improve `tree-iterator` for `harmony`.
* `estraverse` is friendly, powerful and well-tested.
* `estraverse` works faster.

Fixes #816
is this supported for estraverse?