Internal edge class for triangulation.
vL: Null<Vertex2>
vR: Null<Vertex2>
vB: Null<Vertex2>
vT: Null<Vertex2>
kind: EdgeKind
triA: Null<Triangle>
triB: Null<Triangle>
isActive: Bool
nextE: Null<Edge>
prevE: Null<Edge>
new(): Void