Beef up isPredTy
isPredTy can be called on ill-kinded types, especially (of course) if there is a kind error. We don't wnat it to crash, but it was, in piResultTy. This patch introduces piResultTy_maybe, and uses it in isPredTy. Ugh. I dislike this code. It's mainly used to know when we should print types with '=>', and we should probably have a better way to signal that.
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