slight performance increase for graph.all_nodes()
slight performance increase for graph.all_nodes()

set.update() seems to be faster than union and we can save the set
creation overhead:

```python
In [1]: %timeit s = set(range(1000)) ; s.update(range(10000))
1000 loops, best of 3: 419 µs per loop

In [2]: %timeit s = set(range(1000)) ; s.update(set(range(10000)))
1000 loops, best of 3: 584 µs per loop

In [3]: %timeit s = set(range(1000)) ; s.union(set(range(10000)))
1000 loops, best of 3: 610 µs per loop
```

armandobs14/rdflib,avorio/rdflib,yingerj/rdflib,ssssam/rdflib,RDFLib/rdflib,dbs/rdflib,avorio/rdflib,marma/rdflib,armandobs14/rdflib,armandobs14/rdflib,RDFLib/rdflib,yingerj/rdflib,yingerj/rdflib,marma/rdflib,marma/rdflib,ssssam/rdflib,yingerj/rdflib,dbs/rdflib,armandobs14/rdflib,avorio/rdflib,ssssam/rdflib,dbs/rdflib,dbs/rdflib,avorio/rdflib,RDFLib/rdflib,ssssam/rdflib,marma/rdflib,RDFLib/rdflib