Add MoveIdentifier class
MoveIdentifier: Improve documentation.
CoordinateHelpers #empty_square? and documentation.
Move #can_take? to Piece from Pawn.
Add colors class method to Piece.
Use impossible_coordinate factory.
Rename Piece#can_take? to #opponent? and add spec.
Piece#ally? method.
CoordinateSet class with #without_blocks method.
CoordinateSet#each and #empty? and Enumerable
BoardCoordinate#relative_coordinate_set now returns a CoordinateSet.
CoordinateCollection class.
CoordinateCollection: Allow initialization with sets and coordinates.
Pawn#raw_directional_moves returns a CoordinateCollection.
Change BoardHelper method names.
Factories: BoardCoordinates and CoordinateCollections.
King#raw_directional_moves returns a CoordinateCollection
Queen#raw_directional_moves now returns a CoordinateCollection
Rook#raw_directional_moves now returns a CoordinateCollection
Knight and Bishop #raw_directional_moves return CoordinateCollections.