Rollup merge of #57685 - pthariensflame:enhancement/pin-impl-applicability, r=without...

Rollup merge of #57685 - pthariensflame:enhancement/pin-impl-applicability, r=withoutboats

Enhance `Pin` impl applicability for `PartialEq` and `PartialOrd`.

This allows for comparing for equality or ordering a `Pin<P>` and a `Pin<Q>` as long as `P` and `Q` are correspondingly comparable themselves *even when `P` and `Q` are different types*.
An example might be comparing a `Pin<&mut OsString>` to a `Pin<&mut PathBuf>`, which might arise from pin projections from a pair of larger contexts that aren't `Unpin`.