Topology

[zarch]

I think that would be nice to have a set of topological types like:

• Node, with information on: lines that are going in/out of the node, the angle between the lines
• Edge/Arc, with information on: start/end node, next left/right face
• Face, with information on edges that constitute the border, isles and centroid

Then these topological types an be used in rust-topojson and in other crates.
Again I don't know if this topology types should be included in the rust-geo crate or should be moved to a new crate (rust-topology)?
What do you think?