true and nil can be casted to string now

ml.cpp

@@ -287,6 +287,10 @@ MlValue MlValue::cast_to_string() const {
 			return MlValue::string(std::to_string(stack_data.f));
 		case STRING:
 			return *this;
+		case TRUE:
+			return MlValue::string("#t");
+		case NIL:
+			return MlValue::string("nil");
 		default:
 			throw MlError(*this, MlEnvironment(), BAD_CAST_STRING);
 	}