Can we use owning_ref to avoid some unsafe blocks?

[martinvonz]

In a review of unsafe code at Google, the suggestion to use owning_ref (https://docs.rs/owning_ref/latest/owning_ref/) came up. We should look into that.

[arxanas]

owning_ref seems unsound and unmaintained: Kimundi/owning-ref-rs#77. There might be a maintained solution, or it might simply be possible (without having looked at the code) to shuttle around the value and its reference separately in an unergonomic way.

[yuja]

Mercurial recently switched to self_cell, and I think any of these can be used to remove unsafe from recursive tree iterators.
https://github.com/Voultapher/self_cell#related-projects