Detect draws by 50-move-rule (and possibly 3-fold repetition)

[mateon1]

Currently, is_game_over only detects checkmates, stalemates, and insufficient material, but not draws by the 50 move rule, or by 3-fold repetition.

Implementing the 50 move rule should be trivial, as the Chess struct keeps track of the halfmoves counter.
(should also probably change legal_moves and is_legal to match this behavior.)

It's impossible to check for 3-fold repetition in is_game_over given the way the Chess struct is implemented, but maybe it's possible to expose a separate API, that given a starting position and a list of moves, returns whether any position repeated three times.
(I'm sure there's a better solution than keeping a hashmap of board FENs since the last irreversible move)

[Mk-Chan]

Technically the FIDE rules state that a draw is only forced at 75 reversible moves (if not check mate at the final move) and if the same position has appeared 5 times on consecutive alternative moves.

Article 9.6

[niklasf]

With ZobristHash from #40 we now have a useful building block.

Nonetheless I am hesitating on this one, because it requires adding a representation for games (rather than just position), and it seems the best structure is application dependent. Should it be a stack? Should it be a tree? Only the final position with some book keeping?