`impl PartialEq for Option` forces both sides to contain the same `T`

The current `impl PartialEq for Option` looks like this:

```rust
impl<T> PartialEq<Option<T>> for Option<T> where
    T: PartialEq<T> { ... }
```

It could be more general and look like this instead:

```rust
impl<A, B> PartialEq<Option<A>> for Option<B> where
    A: PartialEq<B> { ... }
```