Replace nested array of all moves with a move generator in each piece

[MarkZH]

Experiment

Instead of a large nested array of every possible move, each piece should have a method that generates all moves from a set of steps ((+-1, +-1) for bishops, for example). This will require if(is_pawn()) and if(is_king()) checks for special moves.

[MarkZH]

No need for if(is_pawn()) or other checks. The step moves are just normal moves. Add an is_single_step() method to Move class to tell move creation iterator whether a move can be repeated (for long-range sliding moves like rook and bishop moves) or not (knight, king, pawn).

[MarkZH]

Now that all moves are represented as one Move class [1], this should be easier to implement.

[1] 0e096a5, 3e4e7ff, 5294780, c1425d7, 8cca3f0, 276083c, d53457c

[MarkZH]

Each piece should have two sets of moves: sliding moves that get repeated until the edge of the board is reached, and other moves (pawn moves, king moves, knight moves, castling, etc). The sliding moves are one-square moves that generate all other by repeating, and other moves are just added to the move list if they are legal.