core: add callback to VM context

pkg/vm/context.go

@@ -45,6 +45,14 @@ type Context struct {
 	// Call flags this context was created with.
 	callFlag smartcontract.CallFlag
 
+	// InvocationState contains expected return type and actions to be performed on context unload.
+	InvocationState
+}
+
+// InvocationState contains return convention and callback to be executed on context unload.
+type InvocationState struct {
+	// Callback is executed on context unload.
+	Callback func(ctx *Context)
 	// CheckReturn specifies if amount of return values needs to be checked.
 	CheckReturn CheckReturnState
 }