Safer API for RwIter

[A-Manning]

Pull Request

What does this PR do?

This PR introduces a safer API for RwIter, as a lending iterator.

It is not possible to keep a reference to the key or data at the current cursor after deleting or inserting at the current cursor. If one chooses not to insert or delete at the current cursor, it is possible to keep a reference to the key or data until the end of the current transaction.
It is only possible to insert at the current cursor with the exact key for the current cursor, which is always owned when inserting. It is not possible to insert at the current cursor after deleting at the current cursor, although it is possible to insert multiple times at the current cursor. It is only possible to insert or delete at the current cursor once the cursor has been initialized via a call to RwIter::next or RwIter::last.

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• Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
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[A-Manning]

CI is failing due to unrelated issues

[Kerollmops]

Hey @A-Manning 👋

Thank you very much for this work. It's been a long time since I wanted to improve the usage of the RwIter iterator. I'm sorry again for not getting back to you sooner. Please note that I'll need to take a bit more time to review this, as I am currently working on other features to enhance Meilisearch's indexation speed.

[Kerollmops]

Hey @A-Manning 👋

I rebased your PR on top of main, and as we can see, the unrelated errors disappear. However, we can also see that the code you added is not compiling. Could you please fix it?

Have a nice day 🌵

[A-Manning]

@Kerollmops Thanks, I've fixed the doctests