Auto merge of #139114 - m-ou-se:super-let-pin, r=davidtwco

Implement `pin!()` using `super let`

Tracking issue for super let: rust-lang/rust#139076

This uses `super let` to implement `pin!()`.

This means we can remove [the hack](rust-lang/rust#138717) we had to put in to fix rust-lang/rust#138596.

It also means we can remove the original hack to make `pin!()` work, which used a questionable public-but-unstable field rather than a proper private field.

While `super let` is still unstable and subject to change, it seems safe to assume that future Rust will always have a way to express `pin!()` in a compatible way, considering `pin!()` is already stable.

It'd help [the experiment](rust-lang/rust#139076) to have `pin!()` use `super let`, so we can get some more experience with it.

Author: bors