Transparently handle transformations in tracing

[mmarx]

After #773 is implemented, we should trace rule origins back towards the untransformed program.

[mkroetzsch]

Yes, but I don't see why this has to be strictly after #773. As it was conceived, the transformation origin system should also be usable when origin information is incomplete (as it might always be in this extensible transformation system). Of course, there should be some working cases to try it out on ... but it would actually be good to ensure that the back-transformation works with imperfect origin data as well.