Implementation of Revert functions

[thisiswhereitype]

Alga, Containers and FGL are all good candidates for revert functions. They have functions to allow extraction of edges and nodes in a similar pattern.
This could be a good candidate for a ToPangraph typeclass.

[snowleopard]

I think the primary focus of this library should be efficient parsing/serialisation of various textual/binary graph representations. Once you have a Pangraph a, it's easy to convert it to any library you like.

I would really like to see support of Workcraft graph format, for example.