underAttack

[dadhia]

• Implemented underAttack() which checks whether a location can be moved into by any piece of an opposing color
• Added GetAttackableMoves() to the Piece interface (less strict than GetMoves in that the move is considered attackable even if there is a piece of the same color in the end location)
• For Bishop, Queen, and Rook, this involved the creation of a new function called exploreMoves which limited the amount of repeated code between the GetMoves and GetAttackableMoves function which were inherently quite similar
• For Pawn, I reorganized some code which allowed for a similar approach to Bishop, Queen, and Rook, but at a smaller scope.
• Added a function called CheckLocationForAttackability in piece.go which is a less restrictive function than CheckLocationForPiece

Additional Notes:

• GetAttackableMoves() for King is not the cleanest and repeats some code from GetMoves(), but I can refactor this if necessary once my other pull request (#3) is merged into master
• Will change method signature of underAttack() to allow for multiple locations. This will improve performance as we can check all moves/castling in one function call and one search of the whole board. An alternative is to cache the slice of attacking moves. We can discuss this and make a new branch for this change.

[dadhia]

will reopen when knight and pawn are completed.

[dadhia]

none of this has been tested. I don't know how to test in Go lol. Also, we may need to discuss a way to optimize calls to this on the same game board (either group requests or cache the board).