Refactor ConDecl

The ConDecl type in HsDecls is an uneasy compromise. For the most part, HsSyn directly reflects the syntax written by the programmer; and that gives just the right "pegs" on which to hang Alan's APIannotations. But ConDecl doesn't properly reflect the syntax of Haskell-98 and GADT-style data type declarations.

To be concrete, here's a draft new data type

data ConDecl name
  | ConDeclGADT
      { con_names   :: [Located name]
      , con_type    :: LHsSigType name  -- The type after the ‘::’
      , con_doc     :: Maybe LHsDocString }

  | ConDeclH98
      { con_name    :: Located name

      , con_qvars     :: Maybe (LHsQTyVars name)
        -- User-written forall (if any), and its implicit
        -- kind variables 
        -- Non-Nothing needs -XExistentialQuantification

      , con_cxt       :: Maybe (LHsContext name)
        -- ^ User-written context (if any)

      , con_details   :: HsConDeclDetails name
          -- ^ Arguments

      , con_doc       :: Maybe LHsDocString
          -- ^ A possible Haddock comment.
      } deriving (Typeable)

Note that

• For GADTs, just keep a type. That's what the user writes. (NB: HsType can represent records on the LHS of an arrow: { x:Int,y:Bool } -> T
• con_qvars and con_cxt are both Maybe because they are both optional (the forall and the context of an existential data type
• For ConDeclGADT the type variables of the data type do not scope over the con_type; whereas for ConDeclH98 they do scope over con_cxt and con_details.

This ticket is just to track the thought and invite feedback.