[haskell-llvm] move to mtl-2.0

[Henning Thielemann]

I like to move 'llvm' to transformers/mtl-2. Any reasons that might 
prevent me from doing so?

I would also like to make the version bounds of the package dependencies 
a bit stricter. The llvm package does no longer work with mtl-2, since the 
State constructor is gone, but the Cabal file pretends it would work.