Add Zero and One to Data.Functor (mirroring V1, U1)

[Baldur Blöndal]

Deadline of 2 weeks.

Original discussion: ⁽¹⁾

data Zero a
data One  a = One

Same definitions as in ‘trivia’ package,⁽²⁾
if a consensus is reached we can update
the documentation for Free:⁽³⁾

“Free Zero” is isomorphic to Identity.
“Free One” is isomorphic to Maybe.

⁽¹⁾ https://ghc.haskell.org/trac/ghc/ticket/13177
⁽²⁾ https://hackage.haskell.org/package/trivia
⁽³⁾
https://hackage.haskell.org/package/free-4.12.4/docs/Control-Monad-Free.html
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