FEAT: double ended and exact size traits for Graph iterators
FEAT: double ended and exact size traits for Graph iterators

Add some simple missing trait implementations for graph's iterators.

- DoubleEndedIterator for NodeReferences
- ExactSizeIterator for NodeReferences, EdgeReferences, NodeIndices,
  EdgeIndices

petgraph/petgraph,bluss/petulant-avenger-graphlibrary