release-haskell.nix is intended to be a replacement for
https://github.com/peti/ci/blob/master/haskell-nixpkgs.nix
which is currently the main expression for the haskell-updates jobset
on hydra (in the nixpkgs project).

It has the same jobs as the old haskell-nixpkgs.nix file:

* haskellPackages.*
* haskell.compiler.*
* Some extra haskell packages for certain compilers

The following jobs are new:

* tests.haskell.*
* A manually maintained list of top-level haskell packages (most of them
  using justStaticExecutables)
* An aggregate job which is intended to aid merging the haskell-updates
  branch: It holds an arbitrary list of haskell-related packages and
  tests we intend have working at all times. This is still somewhat
  incomplete and should be extendend in the future.

Additionally a lot of refactoring has been done and some unnecessary
code has been eliminated. Due to the increased set of jobs and my
ideas of convenience however, the code size has grown overall.
I've tried document the individual parts and would be happy about
feedback in general.

One future improvement could be making adding top-level haskell packages
more convenient and adding them all to the aggregate job automatically.