Refactor HsForAllTy

I got sucked into refactoring HsForAllTy, like this:

data HsType name
 = HsForAllTy (HsForAll name) (LHsType name)
 | ...

data HsForAll name      -- The quantifiers of a HsForAllTy
  = HSF { hsf_flag :: HsExplicitFlag
        , hsf_extra :: Maybe SrcSpan  
        , hsf_qtvs :: LHsTyVarBndrs name
        , hsf_ctxt :: LHsContext name
    }

This makes a number of functions neater, as well as naming the fields of a troublesomely-high-arity data constructor.

This ticket is here to track progress.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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