Support "doc-hidden-sealed" trait analysis and related lints

[obi1kenobi]

A sealed trait is one that absolutely cannot be implemented by a downstream crate.

A doc-hidden-sealed trait is one that shouldn't be implemented by a downstream crate, as the implementation would require using or implementing #[doc(hidden)] items. This means downstream crates are technically able to implement such traits, but such implementations rely on non-public API and may be broken without notice in non-major releases.

That means the space of traits is split into three disjoint sets: sealed, doc-hidden-sealed, and non-sealed.

Instances of the problem addressed here are: #997, #953.

We need to:

• work out the exact rules for when a trait is doc-hidden-sealed
• figure out how to reframe existing lints and add new lints to work together with this