isEdge and edgeID methods are suboptimal

[WebDrake]

The current implementations of isEdge and edgeID use a linear search. This should be replaced by a binary search that takes advantage of the sorting of head and tail indexes.

[WebDrake]

Probably the best way to fix this is via a solution to https://d.puremagic.com/issues/show_bug.cgi?id=11667 after which it should be possible to use std.algorithm.find.