Hello!

I recently (while trying to merge two StableGraphs) wanted to transform a StableGraph into an iterator (or a vec) of its NodeType. Unless I fully missed it, this is not possible with the current Api, unless one converts the StableGraph into a normal Graph first, which invalidates all NodeIndex and EdgeIndex.

Since anyone using a StableGraph is storing the indices somewhere (why else would they be using a StableGraph), this is just not an option.
My first solution included cloneing every Node, but that is not an option for everyone (and doubled the peak memory consumption of my program).

I then resorted to just forking petgraph and adding a simple implementation:

pub fn into_nodes_edges(self) -> (impl Iterator<Item = (NodeIndex<Ix>, N)>, impl Iterator<Item = (EdgeIndex<Ix>, NodeIndex<Ix>, NodeIndex<Ix>, E)>) {
    let (inner_nodes, inner_edges) = self.g.into_nodes_edges();

    (
        inner_nodes.into_iter().enumerate().filter_map(|(i, node)| node.weight.map(|node| (i, node))).map(|(index, node)| (NodeIndex::new(index), node)), 
        inner_edges.into_iter().enumerate().filter(|(_, edge)| edge.weight.is_some()).map(|(index, edge)| (EdgeIndex::new(index), edge.source(), edge.target(), edge.weight.unwrap()))
    )
}

This kind of API gives the user all the information that is stable and visible as a consumer of StableGraph, without exposing implementation details AFAIK.