Serialization.vala: stop pretending that the only nodes deserialize_object...

Serialization.vala: stop pretending that the only nodes deserialize_object might get are Documents, but handle getting either a Document or an Element node.  serialize_object () needs to return a Document, as if it tried to just return a an object from one, its owner document would cease to be in scope and its memory would be freed.  However, because a serialization document might contain multiple objects in a hierarchy (like with a collection), anyone implementing Serializable who wants to override how a collection deserializes its members would need to be able to call deserialize_object () on individual nodes (representing the members) and wouldn't have a Document for each one (since they're all in the collection's document)