Warn on types that are `PartialEq<Self>` (`Debug`, `Display`, `PartialOrd<Self>`, `Hash`) but don't expose `dyn PartialEq<dyn Dyncast>` (…)

[Tamschi]

Is your feature request related to a problem? Please describe.

Currently, it's easy to forget to expose a dyncast target for existing functionality, and this can be a bit tricky to debug.

Describe the solution you'd like

The most clear solution would be an additional attribute #[not_dyncast(…)] that accepts (token literal) types for which there'd otherwise be a warning.

Describe alternatives you've considered

An alternative would be to allow negative targets (i.e. dyn !Trait), but that's less obvious and potentially leads to parsing difficulties once the never-type is available.

Additional context

Specifying a literal type with not_dyncast but also exposing it should be statically checked and turned into a compile-time error.

[Tamschi]

A few hints for Hacktoberfest:

• This probably isn't great for beginners at Rust. I'd say it's intermediate and up.
• You can look at https://github.com/Tamschi/Asteracea/blob/f612f5d9981cfe4079db6e8fb6ada05f972b06df/src/__/errors.rs and https://github.com/Tamschi/Asteracea/blob/f612f5d9981cfe4079db6e8fb6ada05f972b06df/proc-macro-definitions/src/part/event_binding.rs#L279-L323 for how to have a macro generate warnings based on Rust type resolution.
• Note that correct Span mapping is required (but I'll help with that if necessary, it's just that it's super useful).
• Please ask if anything remains unclear!

[Tamschi]

Hacktober Hint: I also accept PRs that solve part of the issue here, as long as at least one case is thoroughly covered.