The geometry graph is where the layout's dots, segs, and bends are
stored. The connectivity graph is where information about connected
components and relations between tem (e.g. which ones are connected with
bands) and information common to all band primitives are stored.
This removes dependency on the Slab library. We may go back to it
sometime in the future if we decide to phase out usage of Petgraph's
`StableDiGraph`.
Mikolaj Wielgus