Add OrderedByAwareEffectiveStatement

A number of downstream users end up inquiring ordering of a leaf-list and list nodes. To make their life easier, provide a common OrderedByAwareEffectiveStatement, exposing an ordering() method, which performs the search.

This will allow us to remove centralize the definition of (Leaf)ListSchemaNode.isUserOrdered() and define a default implementation – and remove UserOrderedMixin in future, once we do not rely on isUserOrdered() being fast.