Specified quantification of RuntimeRep for function arrow (->)

Separating specifying quantifiers of (->) !3132 (closed) into this ticked.

I wanted to make sure quantification was specified explicitly, rather than inferred. One of those was the function arrow so I changed the implementation to

(->) :: forall (rep1 :: RuntimeRep) (rep2 :: RuntimeRep). TYPE rep1 -> TYPE rep2 -> Type 

so one could write (->) @LiftedRep @LiftedRep.

Richard objected:

This change is more controversial than you might think. As -> evolves, it may gain all manner of extra arguments. Keeping these Inferred helps protect backward compatibility. Of course, changing to Specified helps those who want to specialize. I see both sides of the story.

More immediately, this may spell trouble for changes coming with linear types. @monoidal do you have any thoughts here?

!3132 (comment 267184)

so I will remove it from the merge request. I will point out that in the documentation (grepping (->) :: under compiler/) the type of (->) is mentioned in four places with a specified quantification. Should they be changed to inferred

-- | The @(->)@ type constructor.
--
-- @
-- (->) :: forall  {rep1 :: RuntimeRep} {rep2 :: RuntimeRep}.
--         TYPE rep1 -> TYPE rep2 -> Type
-- @