[Piece] Implement stream operator

Piece.cpp

@@ -50,7 +50,12 @@ bool operator==(const Piece &lhs, const Piece &rhs) {
 }
 
 std::ostream &operator<<(std::ostream &os, const Piece &piece) {
-    (void) piece;
+    int typeNum = static_cast<int>(piece.type());
+    if (piece.color() == PieceColor::Black) {
+        typeNum = std::tolower(typeNum);
+    }
+
+    os << static_cast<char>(typeNum);
     return os;
 }