refactor: remove Subarray.foldl and other slice operation aliases

[datokrat]

This PR removes Subarray.foldl(M), Subarray.toArray and Subarray.size in favor of the Std.Slice-namespaced operations. Dot notation will continue to work. If, say, Subarray.size is explicitly referred to, an error suggesting to use Std.Slice.size will show up.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2026-02-07 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. You can force Mathlib CI using the force-mathlib-ci label. (2026-02-11 16:54:04)

[leanprover-bot]

Reference manual CI status:

• 🟡 Reference manual branch lean-pr-testing-12441 build against this PR didn't complete normally. (2026-02-11 16:59:46) View Log
• 💥 Reference manual branch lean-pr-testing-12441 build failed against this PR. (2026-02-11 17:01:18) View Log
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• ✅ Reference manual branch lean-pr-testing-12441 has successfully built against this PR. (2026-02-11 20:09:45) View Log
• 🟡 Reference manual branch lean-pr-testing-12441 build against this PR didn't complete normally. (2026-02-11 20:11:12) View Log

[datokrat]

@david-christiansen Given that we can now use suggest_for for a smoother migration experience, I decided to give the removal of Subarray.foldl in favor of Std.Slice.foldl another chance. The presence of both is confusing during verification. Do you agree that suggest_for makes this change viable?

At least my experience of resolving breakage inside Lean was quite nice; I was able to resolve the problems by clicking on the hints in the error messages.