impl Debug for Shared and Unique?

[SimonSapin]

The Shared and Unique types have #[allow(missing_debug_implementations)]. Is there a reason for this? It seems that an impl like write!(formatter, "Shared({:p})", self.as_ptr()) would be more useful.

CC @rust-lang/libs

[frewsxcv]

Opened a PR for this: #46792