Fix supplementary reuse bug by ensuring we track the right view instance in our displayed views cache

[kyleve]

This PR fixes a bug introduced in #288 (well sort of, 288 uncovered the actual bug):

A long time ago, a method argument was renamed from view to anyView, which meant we were no longer shadowing the view variable on the Delegate:

        func collectionView(
            _ collectionView: UICollectionView,
                                         // Was renamed from `view`.
            willDisplaySupplementaryView anyView: UICollectionReusableView,
            forElementKind kindString: String,
            at indexPath: IndexPath
            )
        {
            let container = anyView as! SupplementaryContainerView
            let kind = SupplementaryKind(rawValue: kindString)!
            
            let headerFooter : PresentationState.HeaderFooterViewStatePair = ...

            headerFooter.willDisplay(view: container)

          - self.displayedSupplementaryItems[ObjectIdentifier(view)] = headerFooter
          + self.displayedSupplementaryItems[ObjectIdentifier(container)] = headerFooter

So the fix here was to properly pass the ObjectIdentifier of the container, not view (self.view is actually the ListView).

Without this change, didEndDisplay() was never invoked on the header/footer (because displayedSupplementaryItems is used to track what is being used by the collection view), meaning we never cleared out values during view reuse, which broke the next time a view was reused if the view type was different.

[kyleve]

To be fixed after; but found this when debugging. It's theoretical; but I think it can happen.

[kyleve]

This is the fix! Grr.

[kyleve]

Since this is now weak; need a strong ref to the header.

[kylebshr]

LGTM - will let you decide if the non-necessary changes should go in this PR