Convert `PDFPageViewBuffer` to use a `Set` internally
Convert `PDFPageViewBuffer` to use a `Set` internally

This relies on the fact that `Set`s preserve the insertion order[1], which means that we can utilize an iterator to access the *first* stored view.

Note that in the `resize`-method, we can now move the visible pages to the back of the buffer using a single loop (hence we don't need to use the `moveToEndOfArray` helper function any more).

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[1] This applies to `Map`s as well, although that's not entirely relevant here.

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