Change `Dataset.__iter__` to match RDF spec
Change `Dataset.__iter__` to match RDF spec

Also change Graph/Context identifier annotations to match RDF spec.

Graph/Context identifiers should be URIRef or BNode - in RDFLib these
are now the only extant subclasses of IdentifiedNode, so I'm using that.

RDFLib/rdflib,RDFLib/rdflib,RDFLib/rdflib,RDFLib/rdflib