Better walk upstream

[CJ-Wright]

Currently the walk upstream system creates the graph and walks simultaniously. It might be better to do this in two steps, first find all the nodes between a SimpleToEventStream node and all the associated SimpeFromEventStream node then we can build the graph from that set (see the new rapidz graph building code). This would allow us to catch the edge order of the pipeline in the graph, which is crucial for a proper merkel tree.