Rollup merge of rust-lang#138799 - joboet:extend-tuple, r=Mark-Simulacrum

core: simplify `Extend` for tuples

This is an alternative to rust-lang#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 rust-lang#137400). Additionally, the 1-tuple is now special-cased to allow taking advantage of the well-optimized `Extend` implementations of the individual collection.

Author: matthiaskrgr