Piece examples: Replace alternate_color with piece.opposite_color
Clean up factories.
EmptySquare spec - use empty square factory.
Bishop Spec: Use game and board factories.
Pieces - use factories to build objects.
Aesthetic spec refactors.
BoardCoordinate factories: Allow initialization with x & y, file & rank.
Spec refactors

- :empty trait for coordinate sets
- more use of factories
- be_impossible_coordinate matcher
Change references to BoardCoordinate to Coordinate.
change board_coordinate factory to coordinate
Spec refactors

- More use of factories.
- Remove duplication.
CoordinateCollection#without_blocks