recursive_set

recursive_set method is defined and implemented for NodeMut. This method provides an expressive way to update the values of a tree where value of a node is a function of its prior value and values of its children. Since the values of its children subsequently depend on their own children, it immediately follows that the value of the node depends on values of all of its descendants that must be computed to be able to compute the node's value.

In addition mutable_recursive_traversal example is created to demonstrate different ways to approach to this problem.

Fixes 159.