Generators with UnpinCell

The concept of `UnpinCell` is introduced as a way to create a more ergonomic interface for pinning in Rust, enabling users to convert a pinned mutable reference into a mutable reference. This facilitates generators to support self-referential values, addressing the limitations with the current `Iterator` trait. The post also discusses potential problems with coherence and reverse bridging of iterators and generators and proposes solutions to ensure backward compatibility and ease of use in Rust.