API Massaging

; Tree Traversal

: Callback(node, stack, [acc]); //with this = thisArg.
: Removed some code duplication.



; .reduceChild()
: Instead of creating a new node, empty the current node,
  then iterate on the detached children list.



; Optimized .empty() use case
: Used this optimization to detach children first in mapChild,
  filterChild, which re-attaches children based on return value.

: As a result, node cloning no longer necessary.