refactor: move Iter and others from Std.Iterators to Std

[datokrat]

This PR moves many constants of the iterator API from Std.Iterators to the Std namespace in order to make them more convenient to use. These constants include, but are not limited to, Iter, IterM and IteratorLoop. This is a breaking change. If something breaks, try adding open Std. If some constants in Std.Iterators cannot be found, they can be found directly in Std now.

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2025-12-01 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2025-12-02 09:56:30)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 5339c47555cb65958259b1c4c4c687c3863a08be --onto 646df6ba16b3f68de539a085456c323e160e206d. You can force reference manual CI using the force-manual-ci label. (2025-12-13 00:29:14)

[leanprover-community-bot]

Mathlib CI status (docs):

• 💥 Mathlib branch lean-pr-testing-11446 build failed against this PR. (2025-12-02 10:01:18) View Log
• ✅ Mathlib branch lean-pr-testing-11446 has successfully built against this PR. (2025-12-02 20:06:02) View Log
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 5339c47555cb65958259b1c4c4c687c3863a08be --onto 19e1fe55f33a8aadc4243da32828b70a4b4677cc. You can force Mathlib CI using the force-mathlib-ci label. (2025-12-13 00:29:13)