[Merged by Bors] - feat(*/prod): `prod_prod_prod` equivs

[eric-wieser]

These send ((a, b), (c, d)) to ((a, c), (b, d)), and this commit provides this bundled as equiv, add_equiv, mul_equiv, ring_equiv, and linear_equiv.

We already have something analogous for tensor_product.

---

Split from #18447

[fpvandoorn]

LGTM

bors d+

[bors[bot]]

:v: eric-wieser can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[eric-wieser]

bors merge

[bors[bot]]

Pull request successfully merged into master.

Build succeeded!

The publicly hosted instance of bors-ng is deprecated and will go away soon.

If you want to self-host your own instance, instructions are here. For more help, visit the forum.

If you want to switch to GitHub's built-in merge queue, visit their help page.

• Build mathlib
• Lint mathlib
• Lint style
• Run tests