Add generic representation type RepDeep so that DerivingVia can derive instances for LeafTree via Free []

Motivation

Coercing between types that are equal modulo bottoms like Bool and Either () () doesn't work, because the possible values differ:

For Either () (): ⊥, Left ⊥, Right ⊥, Left (), Right ()
For Bool: ⊥, False, True

Or also for data LeafTree a = Leaf a | Branch [LeafTree a], which can't be coerced to Free [] a.

Proposal

Add generic representation types RepDeep and RepDeep1 to GHC.Generics, which recursively evaluates the representations of all pointers. Then users can write

data LeafTree a = ... deriving (Applicative, Monad) via LeafTree `GenericallyDeep1As` (Free []).

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