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Don't enable redundant_clone, it's currently incredibly broken.

Per review: Made minor fixes. Cut redundant_clone. Extracted deny lints to array.

Extra change: Enabled nursery altogether and switched from explicit deny / opt-in to explicit allow / opt-out. I think this gives a clear sense of what exactly is and isn't being run, what could be enabled, which lints are verbose, have problems, etc. It is generally in the spirit of "makes clippy eat what it produces". But of course, I can revert it too.

While there may not be a lot of FPs now, I suspect these lints could become annoying later down the road, when we get bitten by FPs that "break" ci when some of the clippy code changes in the future.

Okay, reverted the nursery change. Back to opt-in explicit individual lint enables.

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.