Update itertools to 0.15

[jakubadamw]

Update the version of the dependency and make the necessary changes related to the breaking API changes.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @sayantn (or someone else) some time within the next two weeks.

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @Amanieu, @adamgemmell, @davidtwco, @folkertdev, @sayantn
• @Amanieu, @adamgemmell, @davidtwco, @folkertdev, @sayantn expanded to Amanieu, adamgemmell, davidtwco, folkertdev, sayantn
• Random selection from Amanieu, adamgemmell, davidtwco, folkertdev, sayantn

[jakubadamw]

itertools has already been updated to 0.15 in rust-lang/rust, rust-lang/rust-clippy and rust-lang/rust-analyzer.

[jakubadamw]

@sayantn, this failure seems unrelated, am I right? It seems to be blocking a few other PRs as well.

https://github.com/rust-lang/stdarch/actions/runs/28703183510/job/85125343263?pr=2181
https://github.com/rust-lang/stdarch/actions/runs/28876662321/job/85653545102?pr=2183

Caused by:
  process didn't exit successfully: `/checkout/target/release/build/mod_0-f7a6d405d2e008b7/build-script-build` (exit status: 1)
[…]
  --- stderr


  error occurred in cc-rs: failed to find tool "/opt/intel/oneapi/compiler/2026.0/bin/icx": No such file or directory (os error 2)

It looks like a regression from #2125.

[sayantn]

Yes, it looks unrelated. Will take a look