Issue 9231: add cast_integer lint

[gernot-ohner]

fixes #9231

PR checklist:

• Followed [lint naming conventions][lint_naming]
• Added passing UI tests (including committed .stderr file)
• cargo test passes locally
• Executed cargo dev update_lints
• Added lint documentation
• Run cargo dev fmt

changelog: [`cast_integer`]: add lint for all integer casts using `as`

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Alexendoo (or someone else) soon.

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[bors]

☔ The latest upstream changes (presumably #12087) made this pull request unmergeable. Please resolve the merge conflicts.

[xFrednet]

Hey @gernot-ohner, this is a ping from triage, since there hasn't been any activity in some time. Are you still planning to continue this implementation?

If you have any questions, you're always welcome to ask them in this PR or on Zulip.

@rustbot author

[Alexendoo]

Closing this since it's been a while, but if you ever want to pick it up again please feel free!