fdaa91a0d7
Stabilize the `cell_update` feature
Included API:
```rust
impl<T: Copy> Cell<T> {
pub fn update(&self, f: impl FnOnce(T) -> T);
}
```
FCP completed once at https://github.com/rust-lang/rust/issues/50186#issuecomment-2198783432 but the signature has since changed.
Closes: https://github.com/rust-lang/rust/issues/50186