New lint: Recommend using ptr::eq when possible

[ambroisie]

This is based almost entirely on the code available in the previous PR #4596. I merely updated the code to make it compile.

Fixes #3661.

• I'm not sure about the lint name, but it was the one used in the original PR.
• Added passing UI tests (including committed .stderr file)
• cargo test passes locally
• Executed cargo dev update_lints
• Added lint documentation
• Run cargo dev fmt

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changelog: none

[rust-highfive]

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[Manishearth]

@bors r+

[Manishearth]

@bors r+

[bors]

📌 Commit 6edde81 has been approved by Manishearth

[bors]

⌛ Testing commit 6edde81 with merge 947516f...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 947516f to master...