map (callback) {
      return new AccessorNode(
        this._ifNode(callback(this.object, 'object', this)),
        this._ifNode(callback(this.index, 'index', this))
      )
    }
Create a new AccessorNode whose children are the results of calling
the provided callback function for each child of the original node.
@param {function(child: Node, path: string, parent: Node): Node} callback
@returns {AccessorNode} Returns a transformed copy of the node