Generate more edge-handling code in trustfall_stubgen

[obi1kenobi]

We can generate more of the body of edge handler functions. We currently leave a todo!() as the body, but most edges are resolved one-at-a-time and we can make by generating a body like this:

    pub(super) fn generated_edge_fn<'a>(
        contexts: ContextIterator<'a, Vertex>,
        _resolve_info: &ResolveEdgeInfo,
    ) -> ContextOutcomeIterator<'a, Vertex, VertexIterator<'a, Vertex>> {
        resolve_neighbors_with(contexts, |vertex| {
            let vertex = vertex.as_<generated>().expect("not a <VertexType>");
            let neighbors = todo!(...);
            Box::new(neighbors)
        })
    }