Shared data type for extension flags

[Herbert Valerio Riedel]

On 2015-09-02 at 10:00:40 +0200, Matthew Pickering wrote:
> Surely the easiest way here (including for other tooling - ie
> haskell-src-exts) is to create a package which just provides this
> enumeration. GHC, cabal, th, haskell-src-exts and so on then all
> depend on this package rather than creating their own enumeration.

I'm not sure this is such a good idea having a package many packages
depend on if `ghc` is one of them, as this forces every install-plan
which ends up involving the ghc package to be pinned to the very same
version the `ghc` package was compiled against.

This is a general problem affecting packages `ghc` depends upon (and as
a side-note starting with GHC 7.10, we were finally able to cut the
package-dependency between `ghc` and `Cabal`)

Also, Cabal is not GHC specific, and contains a list of known extensions
(`KnownExtension`) across multiple Haskell compilers

  https://github.com/haskell/cabal/blob/master/Cabal/Language/Haskell/Extension.hs

and I assume the extension enumeration needed for GHC would be tailored
to GHC's need and omit extensions not relevant to GHC, as well as
include experimental/internal ones not suited for consumption by
Cabal.