Add Cleanupable interface

The Cleanup method is called on actors when they exit

actor.go

@@ -127,3 +127,10 @@ type RunShutdownable interface {
 	// should be shut down.
 	Shutdown(ctx context.Context) error
 }
+
+// Cleanupable is any object that must be cleaned up after it has stopped for
+// good. Actors which implement this interface will be cleaned up after they
+// are deleted from the environment.
+type Cleanupable interface {
+	Cleanup(ctx context.Context) error
+}