Expose visitor pattern for traversing trees

[tturocy]

This refactors the implementation of traversing the nodes in a tree to expose a full visitor-pattern interface. The existing preorder and postorder traversal modes are now implemented as special cases of the visitor pattern.

This re-implements two algorithms to eliminate recursion by using the visitor pattern:

• Determining if a game is constant-sum
• Determining the minimum/maximum payoff to a player in any play of the game.