deprecate Eq::assert_receiver_is_total_eq and emit FCW on manual impls

[cyrgani]

The Eq::assert_receiver_is_total_eq method is purely meant as an implementation detail by #[derive(Eq)] to add checks that all fields of the type the derive is applied to also implement Eq.
The method is already #[doc(hidden)] and has a comment saying // This should never be implemented by hand..
Unfortunately, it has been stable since 1.0 and there are some cases on GitHub (https://github.com/search?q=assert_receiver_is_total_eq&type=code) where people have implemented this method manually, sometimes even with actual code in the method body (example: https://github.com/Shresht7/codecrafters-redis-rust/blob/31f0ec453c504b4ab053a7b1c3ff548ff36a9db5/src/parser/resp/types.rs#L255).
To prevent further confusion from this, this PR is deprecating the method and adds a FCW when it is manually implemented (this is necessary as the deprecation warning is not emitted when the method is implemented, only when it is called).
This is similar to what was previously done with the soft_unstable lint (#64266).

See also rust-lang/libs-team#704.

[rustbot]

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[cyrgani]

TODO: create a real tracking issue once there is consensus on this change

[madsmtm]

Implementation looks good, feel free to r=me on that once you have t-libs signoff (FCP?).

Procedurally, this might also need t-lang approval for the lint name IIUC? Or can t-libs decide that?

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