New official language extension tokens for GHC 7.8.1

[Herbert Valerio Riedel]

Hello GHC Devs,

Which of the language extensions listed in `expectedGhcOnlyExtensions` in

  http://ghc.haskell.org/trac/ghc/browser/testsuite/tests/driver/T4437.hs#L33

are deemed ready for public consumption in GHC 7.8.1, and can be
registered with upstream Cabal now (maybe even for the upcoming 1.18.0
release)?

Related: http://ghc.haskell.org/trac/ghc/ticket/8176

Cheers,
  hvr