structuralShowsPrec with type patterns

[RyanGlScott]

Most of the functions in Data.Parameterized.TH.GADT (e.g., structuralTypeEquality) take an [(TypePat, ExpQ)] argument to allow customizing the behavior of the TH-generated code in certain cases. There is one exception to this rule, however: structuralShowsPrec¹. This is a shame, because there are definitely situations where being able to have custom Show behavior in certain cases would be handy.

I propose we add this capability. If we don't want to break the existing structuralShowsPrec API, we could always add a variant of structuralShowsPrec that allows this.

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¹Unless you count the deprecated structuralHash function.