Auto merge of #7762 - HKalbasi:master, r=Manishearth

Add lint `equatable_if_let`

This is my attempt for #1716. There is a major false positive, which is people may implement `PartialEq` in a different way. It is unactionable at the moment so I put it into `nursery`.

There is a trait `StructuralPartialEq` for solving this problem which is promising but it has several problems currently:
* Integers and tuples doesn't implement it.
* Some types wrongly implement it, like `Option<T>` when `T` doesn't implement it.

I consider them bugs and against the propose of `StructuralPartialEq`. When they become fixed, this lint can become a useful lint with a single line change.

changelog: New lint: [`equatable_if_let`]

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