Introduces a script that can be used to update the Nix expressions for
the Haskell package set. In service of that, also
- introduces cabal2nix-latest, which pins the hackage2nix version used
- changes all-cabal-hashes to use fetchFromGitHub
- adds update-hackage.sh & update-cabal2nix-latest.sh & update-stackage.sh maintainer scripts
