Review fixes

Co-authored-by: Marin Veršić <marin.versic101@gmail.com>
Signed-off-by: Dmitry Murzin <diralik@yandex.ru>

crates/iroha_data_model/src/transaction.rs

@@ -175,6 +175,7 @@ mod model {
         #[getset(skip)]
         pub value: SignedTransaction,
         /// Reason of rejection
+        // NOTE: Using `Box` reduces memory use by 10%
         pub error: Option<Box<error::TransactionRejectionReason>>,
     }
 }