### What it does
Checks for deriving `Hash` but implementing `PartialEq`
explicitly or vice versa.

### Why is this bad?
The implementation of these traits must agree (for
example for use with `HashMap`) so it’s probably a bad idea to use a
default-generated `Hash` implementation with an explicitly defined
`PartialEq`. In particular, the following must hold for any type:

```
k1 == k2 ⇒ hash(k1) == hash(k2)
```

### Example
```
#[derive(Hash)]
struct Foo;

impl PartialEq for Foo {
    ...
}
```