Make events a list of lists so we can associate events with their ope…

…ration (#59)

* Make events a list of lists so we can associate events with their operation

* Add a OperationEvents struct

Stellar-ledger.x

@@ -412,15 +412,20 @@ struct ContractEvent
     body;
 };
 
+struct OperationEvents
+{
+    ContractEvent events<>;
+};
+
 struct TransactionMetaV3
 {
     LedgerEntryChanges txChangesBefore; // tx level changes before operations
                                         // are applied if any
     OperationMeta operations<>;         // meta for each operation
     LedgerEntryChanges txChangesAfter;  // tx level changes after operations are
                                         // applied if any
-    ContractEvent events<>;            // custom events populated by the
-                                        // contracts themselves
+    OperationEvents events<>;           // custom events populated by the
+                                        // contracts themselves. One list per operation.
     TransactionResult txResult;
 
     Hash hashes[3];                     // stores sha256(txChangesBefore, operations, txChangesAfter),