Considering using niv for nix

[bhoudebert]

Instead of tracking directly a tarball in the nix file it could interesting to use niv.

I really think it will be easier to track version, branch and for updates, what do you think?

Regards.

[MondayMorningHaskell]

My recent experience upgrading the GHC version makes me think this could be a good idea! I opened a draft PR creating the sources files, but I'm not sure what I need to do to import them (I'm still quite unfamiliar with Nix). If you want to pick up on that PR you're welcome to!

#48

(Apologies it took me a while to get to this)