Get dhall-nix on stackage

[domenkozar]

To avoid #9 in the future, we should get dhall-nix to LTS-12.X and nightly again. I'm willing to help on this front and be stackage co-maintainer.

cc @Gabriel439

[Gabriella439]

Yeah, feel free register as a co-maintainer of dhall-nix on Stackage

My understanding is that the main blocker here is getting hnix on Stackage. It used to be there but eventually got removed

[domenkozar]

commercialhaskell/lts-haskell#146

[Gabriella439]

I'm closing out issues before archiving this repository since dhall-nix is moving into the dhall-haskell project.

We're a step closer to getting this on Stackage now that hnix supports the latest megaparsec. There are still some issues with hnix that require building against the GitHub version instead of Stackage, but once it's Stackage-ready I'll add it