Make `PartialOrd for Vec` allocator independent

[gootorov]

Proposal

Problem statement

PartialOrd cannot currently be used on vectors allocated with different allocators.

Motivating examples or use cases

• It is consistent with PartialEq<Vec<U, A2>> for Vec<T, A1>.
• Writing tests for/using data structures that are parameterized by an allocator may be tedious due to the fact that an underlying vector cannot be compared with different allocators.

Solution sketch

See PR: rust-lang/rust#112632

Alternatives

Compare underlying slices via Deref<Target = [T]>.

[dtolnay]

Seems good to me.

Let's do a libs-api team FCP on your PR, since this would need to be instantly stable.

[thomcc]

It's not insta-stable because Allocator isn't stable (FCP seems fine to me either way, just pointing it out).

[dtolnay]

You are right, good call.