Add Tuple.{apply, unapply}

[nicolasstucki]

Part of #9033

[liufengyun]

Looking at the changes alone, it is not obvious that this is an improvement in terms of programming experience.

Maybe also add the other half so that the benefits will be more noticeable.

[nicolasstucki]

To add the next step I need this change in the reference compiler. Otherwise I would need to duplicate some rather large file in the bootstrapped/non-bootstraped library and lose thier history.

It is also orthogonal self contained change compared to the other changes. Hence a good candidate for its own PR.

[liufengyun]

What about pushing the design in another PR (no need to be green), so that we can see the proposal as a whole? Seeing the solution as a whole might consolidate/invalidate some assumptions, or give rise to better ideas.

[nicolasstucki]

This constructor/extractor are useful on their own even if we do not do the rest.