feat(unnecessary_fold): lint on `fold`s with `Add::add`/`Mul::mul`

[ada4a]

Resurrects https://github.com/rust-lang/rust-clippy/pull/13475

changelog: [unnecessary_fold]: lint on folds with Add::add/Mul::mul

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[ada4a]

@rustbot ready

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.