Rollup merge of #83791 - the8472:relax-zip-side-effect-guarantee, r=dtolnay

Rollup merge of #83791 - the8472:relax-zip-side-effect-guarantee, r=dtolnay

Weaken guarantee around advancing underlying iterators in zip

The current guarantee (introduced in #52279) is too strong as it prevents adapters from exploiting knowledge about the iterator length and using counted loops for example because they would stop calling `next()` before it ever returned `None`. Additionally several nested zip iterators already fail to uphold this.

This does not yet remove any of the specialization code that tries (and sometimes fails) to uphold the guarantee for `next()`
because removing it would also affect `next_back()` in more surprising ways.

The intent is to be able to remove for example this branch

https://github.com/rust-lang/rust/blob/36bcf4069717b9dff90270d13b53a3b130329960/library/core/src/iter/adapters/zip.rs#L234-L243

or this test

https://github.com/rust-lang/rust/blob/36bcf4069717b9dff90270d13b53a3b130329960/library/core/tests/iter/adapters/zip.rs#L177-L188

Solves #82303 by declaring it a non-issue.