Implement Haskell CI

[sjshuck]

From @andreasabel:

I notice though that CI does not cover all major GHC versions for mtl. I would recommend to add an autogenerated Haskell CI because this tests on different GHCs with their shipped libraries.
Haskell CI has to be build from the repo https://github.com/haskell-CI/haskell-ci (Hackage is too old and to seldom released), and you have to maintain the tested-with field in the cabal file. I also recommend to have a cabal.haskell-ci with content branches: master to avoid redundant CI runs.