tree: remove unsafe with `ouroboros` for self-referential iterators #7774

Job	Run time
build (ubuntu-latest)	9m 46s
cargo-deny (advisories)	25s
build (macos-13)	5m 1s
Check protos	52s
cargo-deny (bans licenses sources)	22s
build (windows-latest)	6m 45s
Clippy check	2m 11s
build (ubuntu-latest, --all-features)	11m 32s
Check that MkDocs can build the docs	26s
Check that MkDocs can build the docs with Poetry 1.3.2	1m 5s
Check formatting	15s
	38m 40s

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