Proposal: add `on` to the Prelude

[David Feuer]

Every time I reach for Data.Function.on, I feel like a total dolt for
having to import a module to get a function whose implementation is barely
longer than the import. And it's a really good function too! Can we please
add it to the Prelude?

  on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
  (.*.) `on` f = \x y -> f x .*. f y
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