Many clippy lints usually enforced by `-Dcomplexity` and `-Dcorrectness`
are not caught by CI as they are gated by `features`, like
`runtime-benchmarks`, while the clippy CI job runs with only the default
features for all targets.

This PR also adds a CI step to run clippy with `--all-features` to
ensure the code quality is maintained behind feature gates from now on.

To improve local development, clippy lints are downgraded to warnings,
but they still will result in an error at CI due to the `-Dwarnings`
rustflag.

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Co-authored-by: Liam Aharon <[email protected]>