Fix StandaloneDeriving
Fix StandaloneDeriving

If I understand correctly, `deriving instance _ => Eq (Foo a)`
is equivalent to `data Foo a deriving Eq`, rather than
`data Foo a deriving Foo`.
sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc