Add `#[diagnostic::not_object_safe]`

[kornel]

I like the feature, but the term Object Safe is an awful jargon.

I've opened a separate thread to bikeshed about the name:

Object Safety is a terrible term

The term "Object Safety" is like a "Guinea Pig" — not a pig, and not from Guinea. Rust isn't a typical OOP language, so what is an "Object" (in the broad meaning) in Rust is unclear, and could be multiple things depending on one's interpretation. However, Object Safety is not about any objects, it's about traits. It's an abbreviated version of "Trait-Object Safety", but it leaves out the most relevant word! It uses a colloquial meaning of safety unrelated to Rust's primary concept of safe/un…