Eta reduce type synonyms when possible

If you can eta reduce a type synonym, it is always better:

-- Bad
type F a = Maybe a
instance Functor F -- illegal, partially applied type synonym! 
-- Good
type F = Maybe
instance Functor F -- ok with type synonym instances

So... GHC should do this automatically for users. Ordinarily this is not terribly important (thus the priority); in the context of #12680 it is a modestly helpful affordance which allows a user to implement data Map a using type Map a = HashMap a (signature matching requires the type synonym to be nullary.)

Trac metadata

Trac field	Value
Version	8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	lowest
Resolution	Unresolved
Component	Compiler (Type checker)
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CC	goldfire
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