enable stackage for better dependency management

[doofin]

[Yuras]

It's a surprise for me that pdf-toolbox is not on stackage. It definitely was, and I'm not sure what happened.

Right now pdf-toolbox is a state where the stable version is almost dead (but not yet) and the new one is almost done (but not yet). And I'm out of energy to finish it. In this situation I'm not going to invest time into investigating this issue with stackage. But it would be awesome if someone takes maintainance on stackage.

[doofin]

thanks! sorry for my oversight

[Yuras]

Seems to be on stackage already, see e.g. #88