Split llvm-base package

[amigalemming]

I propose to split the llvm-base package into a low-level llvm-ffi package and a mid-level package named llvm-mid or llvm-wrapper. The mid-level interface adds the mtl dependency and is more experimental. The low-level interface should be rather canonical, but needs the difficult configuration process, the HSC preprocessor, but it does not need mtl with functional dependencies. Thus the low-level interface might even run on Haskell compilers without many extensions like JHC. If the packages are split, they shall also be in separate repositories.