Add NormalizedNodeContainer.size()

Binding-DOM codec needs to translate MapNodes into Maps. In order to do that efficiently, it needs to have access to the number of children. While this is available through MapNode.getValue().size(), this is forcing materialization of Map.values() view and an unmodifiable view – neither of which are desirable.

Add NormalizedNodeContainer.size() to allow the implementation to expose this property without intermediate objects being instantiated.