Add TransactionInvocation.GetScript to StateReader (#170)

This allows smart contracts to check that the transaction is
an invocation to the same contract during verification phase.

Example use case: neo-project/proposals#18 (comment)

neo/SmartContract/StateReader.cs

@@ -104,6 +104,7 @@ public StateReader()
             Register("Neo.Transaction.GetOutputs", Transaction_GetOutputs);
             Register("Neo.Transaction.GetReferences", Transaction_GetReferences);
             Register("Neo.Transaction.GetUnspentCoins", Transaction_GetUnspentCoins);
+            Register("Neo.InvocationTransaction.GetScript", InvocationTransaction_GetScript);
             Register("Neo.Attribute.GetUsage", Attribute_GetUsage);
             Register("Neo.Attribute.GetData", Attribute_GetData);
             Register("Neo.Input.GetHash", Input_GetHash);
@@ -671,6 +672,18 @@ protected virtual bool Transaction_GetUnspentCoins(ExecutionEngine engine)
             return false;
         }
 
+        protected virtual bool InvocationTransaction_GetScript(ExecutionEngine engine)
+        {
+            if (engine.EvaluationStack.Pop() is InteropInterface _interface)
+            {
+                InvocationTransaction tx = _interface.GetInterface<InvocationTransaction>();
+                if (tx == null) return false;
+                engine.EvaluationStack.Push(tx.Script);
+                return true;
+            }
+            return false;
+        }
+
         protected virtual bool Attribute_GetUsage(ExecutionEngine engine)
         {
             if (engine.EvaluationStack.Pop() is InteropInterface _interface)