case-insensitive

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It's debatable whether it's a true law of Eq (opinions varied in #haskell), but I'd say it certainly is an expectation that if a == b then f a == f b. The original function breaks this for the CI type.

Relevant discussion on reddit.

Relevant discussion on Haskell Cafe.

@dag as I understand it this invariant is violated by other functions too, specifically show, map and traverse.

I'm tempted to close this as wontfix.