Fix promotion bugs: all test positions from the wiki are now passing

src/Chess/board.nim

@@ -1509,7 +1509,7 @@ proc doMove(self: ChessBoard, move: Move) =
         castlingAvailable = self.position.castlingAvailable
         enPassantTarget = emptyLocation()
     # Needed to detect draw by the 50 move rule
-    if piece.kind == Pawn or move.isCapture():
+    if piece.kind == Pawn or move.isCapture() or move.isEnPassant():
         halfMoveClock = 0
     else:
         inc(halfMoveClock)
@@ -1609,7 +1609,6 @@ proc doMove(self: ChessBoard, move: Move) =
         # Make the en passant pawn disappear
         self.removePiece(move.targetSquare + piece.color.bottomSide(), attack=false)
 
-
     if move.isCapture():
         # Get rid of captured pieces
         self.removePiece(move.targetSquare, attack=false)
@@ -1618,6 +1617,7 @@ proc doMove(self: ChessBoard, move: Move) =
     if move.isPromotion():
         # Move is a pawn promotion: get rid of the pawn
         # and spawn a new piece
+        self.removePiece(move.targetSquare)
         case move.getPromotionType():
             of PromoteToBishop:
                 self.spawnPiece(move.targetSquare, Piece(kind: Bishop, color: piece.color))
@@ -1628,10 +1628,9 @@ proc doMove(self: ChessBoard, move: Move) =
             of PromoteToQueen:
                 self.spawnPiece(move.targetSquare, Piece(kind: Queen, color: piece.color))
             else:
+                # Unreachable
                 discard
     self.updateAttackedSquares()
-    # TODO: Remove this, once I figure out what the heck is wrong
-    # with updating the board representation
     self.updateBoard()