core: simplify `Extend` for tuples
This is an alternative to https://github.com/rust-lang/rust/pull/137400. The current macro is incredibly complicated and introduces subtle bugs like calling the `extend_one` of the individual collections in backwards order. This PR drastically simplifies the macro by removing recursion and moving the specialization out of the macro. It also fixes the ordering issue described above (I've stolen the test of the new behaviour from https://github.com/rust-lang/rust/pull/137400). Additionally, the 1-tuple is now special-cased to allow taking advantage of the well-optimized `Extend` implementations of the individual collection.
Some history: The Zip TrustedRandomAccess specialization has tried
to emulate the side-effects of the naive implementation for a long time,
including backwards iteration. 82292¹ tried to fix unsoundness (82291¹) in that
side-effect-preservation code, but this introduced some panic-safety
unsoundness (86443¹), but the fix 86452¹ didn't fix it for nested Zip
iterators (137255¹).
Rather than piling yet another fix ontop of this heap of fixes this PR reduces
the number of cases in which side-effects will be preserved; the necessary
API guarantee change was approved in 83791¹ but we haven't made use of that
so far.
¹ see merge commit for linkfied issues.
