Soundness review request: selfref 0.2.2

So Rust 1.65.0 brings GATs to stable, as well as some important bugfixes with regards to unsizing/dyn with associated types (which, by extension, also applied to GATs). selfref 0.2.x brings ?Sized bounds to selfref (which is a breaking change from 0.1.x), and 0.2.2 specifically removes the #![feature(generic_associated_types)] (uhh do we call these "feature flags" or...?), allowing it to work on stable.

But we'd like a proper code review now. That seems important at this point.