typeKind is quadratic

While pondering D3998, I realized that any computation of a type's kind should be careful in the presence of nested AppTys. And, to my horror, I realized that GHC's very own typeKind isn't! That is, if you have the type a b c d e, GHC will take a quadratic (or worse -- haven't benchmarked) amount of time computing its kind.

This can be easily fixed by looking for nested AppTys and using piResultTys, instead of repeated uses of piResultTy (as is currently done).

NB: This was not discovered through witnessing poor performance, but it does seem like very low-hanging compiler-performance fruit.

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Trac field	Value
Version	8.2.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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