insert method
Insert a EdgeEnd in order in the list.
If there is an existing EdgeStubBundle which is parallel, the EdgeEnd is
added to the bundle. Otherwise, a new EdgeEndBundle is created
to contain the EdgeEnd.
Implementation
void insert(EdgeEnd e) {
EdgeEndBundle? eb = edgeMap[e];
if (eb == null) {
eb = new EdgeEndBundle.withEdgeEnd(e);
insertEdgeEnd(e, eb);
} else {
eb.insert(e);
}
}