Rename "levity polymorphism" to "representation polymorphism"?

As argued in the paper "Kinds are calling conventions", GHC's notion of "levity polymorphism" should be called "representation polymorphism". This was not an issue for a while, but since #17526 (closed), GHC supports the Levity kind in addition to RuntimeRep.

This ticket is a proposal: let's change the existing references to levity polymorphism (in GHC documentation and comments) to "representation polymorphism", and use the term "levity polymorphism" only for BoxedRep l (where l :: Levity).

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