Add constructor to SerializableAction to make creation from an action easier.

Runtime/SerializableAction.cs

@@ -76,5 +76,17 @@ namespace SerializableFunc
         {
             return serializableAction?.GetAction();
         }
+
+        public SerializableAction()
+        {
+        }
+
+        public SerializableAction(Action action)
+        {
+            if (action == null) throw new ArgumentNullException(nameof(action));
+            targetObject = action.Target as Object;
+            methodName = action.Method.Name;
+            cachedAction = action;
+        }
     }
 }