Packaging haskell libraries

- https://github.com/conda-forge/ghc-feedstock/pull/24

With the above PR, we can now build GHC in such a way that we can compile Haskell programs for all conda-forge platforms except Windows. This would allow us to ship any haskell based binary in conda-forge instead of repackaging a third-party build. There still needs to be some discussion/work on:

- [ ] Write a grayskull(-like) plugin that can generate recipes from cabal files (@xhochy has a draft that fulfills this for 80%)
- [ ] Decide whether we want to link statically or dynamically
- [ ] How to handle the global pinning of Haskell libraries (might be a lot of them). Options include: omit for the start as dependencies are tightly pinned anyways; update the pinning on a weekly basis; just do the same as with other libraries
- [ ] Add a `ghc-activation` recipe that installs the correct activation scripts for cross-compilation
- [ ] Decide a prefix for haskell libraries
- [ ] Do we want to make `@conda-forge/ghc` a maintainer of all haskell packages or should we introduce `@conda-forge/haskell` ?

cc @conda-forge/ghc