Suggest use of associated type bounds where possible

What it does

Now that associated type bounds are stabilized, I'd love to have a clippy lint (ideally with machine-applicable fix) that catches cases that could be written using associated type bounds.

This could start with the simplest cases:

• if a set of bounds contains a trait with an associated type, and has the form T: TraitWithAssoc, <T as TraitWithAssoc>::Assoc: Bound, lint with a suggestion to merge into T: TraitWithAssoc<Assoc: Bound>.
• if a set of bounds contains a trait with an associated type, and has the form T: TraitWithassoc<Assoc = U>, U: Bound, and U isn't used for anything else, lint with a suggestion to merge into T: TraitWithAssoc<Assoc: Bound> and delete the declaration of U.

Advantage

This simplifies bounds and makes them easier to follow, with less indirection. In some cases it also eliminates the need for the complex <T as TraitWithAssoc>::Assoc syntax.

Drawbacks

No response

Example

fn func<T>(iter: T)
where
    T: Iterator,
    <T as Iterator>::Item: Display,

Could be written as:

fn func<T>(iter: T)
where
    T: Iterator<Item: Display>,