Rollup merge of #93755 - ChayimFriedman2:allow-comparing-vecs-with-different-allocato...

Rollup merge of #93755 - ChayimFriedman2:allow-comparing-vecs-with-different-allocators, r=dtolnay

Allow comparing `Vec`s with different allocators using `==`

See https://stackoverflow.com/q/71021633/7884305.

I did not changed the `PartialOrd` impl too because it was not generic already (didn't support `Vec<T> <=> Vec<U> where T: PartialOrd<U>`).

Does it needs tests?

I don't think this will hurt type inference much because the default allocator is usually not inferred (`new()` specifies it directly, and even with other allocators, you pass the allocator to `new_in()` so the compiler usually knows the type).

I think this requires FCP since the impls are already stable.