Player and Action classes are now singletons (factory pattern) rather than String values.
Implementing more general treesearch code for minimax, alpha-beta, monte carlo using simplified backup logic.
Removed unnecessary distinction between policy and tree search (tree search is a special kind of policy).
Calculation of all valid moves / arbitrary sets of moves is now a seperate class, as it serves a different purpose than a policy.
Introduced regression error in AlphaBeta test.
