Rollup merge of #40319 - eddyb:it's-"unsize"-not-"unsound", r=nikomatsakis

Rollup merge of #40319 - eddyb:it's-"unsize"-not-"unsound", r=nikomatsakis

Disallow subtyping between T and U in T: Unsize<U>.

Because `&mut T` can be coerced to `&mut U`, `T` and `U` must be unified invariantly. Fixes #40288.
E.g. coercing `&mut [&'a X; N]` to `&mut [&'b X]` must require `'a` be equal to `'b`, otherwise you can convert between `&'a X` and `&'b X` (in either direction), potentially unsoundly lengthening lifetimes.

Subtyping here was introduced with `Unsize` in #24619 (landed in 1.1, original PR is #23785).