[Board] Implement stream operator

Board.cpp

@@ -71,6 +71,25 @@ void Board::pseudoLegalMovesFrom(const Square &from,
 }
 
 std::ostream &operator<<(std::ostream &os, const Board &board) {
-    (void) board;
+    // For debugging only, performance isn't important
+    for (int i = 63; i >= 0; i--) {
+        // Get the piece for this index. Assume it exists.
+        auto piece = board.piece(Square::fromIndex(i).value());
+
+        // Print piece, otherwise '.';
+        if (piece.has_value()) {
+            os << piece.value();
+        } else {
+            os << '.';
+        }
+
+        // If a file is done, output newline
+        if (i % 8 == 0) {
+            os << '\n';
+        } else {
+            os << ' ';
+        }
+    }
+
     return os;
 }