Restructured Haskell packages a bit.

   * There's a new file called haskell-defaults.nix.
   * The new file contains:

         - Default package version settings for each compiler version.
         - All the stuff that previously still was in all-packages.nix.

   * A relatively small part is left in all-packages.nix.

svn path=/nixpkgs/trunk/; revision=34088