Use deref target in Pin trait implementations

Using deref target instead of pointer itself avoids providing access to
`&Rc<T>` for malicious implementations, which would allow calling
`Rc::get_mut`.

This is a breaking change necessary due to unsoundness, however
the impact of it should be minimal.

This only fixes the issue with malicious `PartialEq` implementations,
other `Pin` soundness issues are still here.

See <https://internals.rust-lang.org/t/unsoundness-in-pin/11311/73>
for more details.

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