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