Check lints related to trait implementations for false-positives if the trait impl is `#[doc(hidden)]`

[obi1kenobi]

For example:

pub struct SecretlyIter;

#[doc(hidden)]
impl Iterator for SecretlyIter {
    type Item = i64;

    fn next(&mut self) -> Option<Self::Item> {
        None
    }
}

The fact that SecretlyIter is impl Iterator is not public API. But there is still a hazard, since users may rely on SecretlyIter being impl Iterator indirectly, e.g. by using it in a for loop. Discussion on that here: https://hachyderm.io/@[email protected]/112292583897196920

This situation will require some nuance. Tentative plan:

• [ ] Add test cases for various edge cases, to pin down the current behavior of our lints.
• [ ] Make the "trait no longer implemented" lint check that the original impl was public API.
• [ ] Add a new lint for "trait impl stopped being public API".
• [ ] After #58, add a new warning-level lint that checks for "trait no longer implemented" ignoring the public API property, narrowed specifically for special traits in the Rust prelude like Iterator.
• [ ] Eliminate any other false-positives, and consider adding more lints if needed, analogously to the above.