Fix Segment creation
Sanity check; we just aren't going to deal with the pathological case of no edges
Refine the making of FlatGraphs

It turned out that we needed both Edges and Segments to be able to
create blocks later in the process, so I reworked the FlatGraph
code to include them. This removed the need to transform :preds
and :succs into specifically flat versions. I also brought :marked
along for the ride; not including it originally was an oversight.