`MainThreadOnly` on generic classes

[madsmtm]

NSCollectionViewDiffableDataSource isn't MainThreadOnly, so does not implement NSCollectionViewDataSource since that has MainThreadOnly as a super trait.

I'm not entirely sure to resolve that? Maybe generic types need to implement MainThreadOnly if all their generics do? Though this would also allow NSArray<NSVIew> to be MainThreadOnly, which isn't desirable, since that would allow <NSArray<NSView>>::new().mtm().

See also discussion on Matrix.