index: turn CompositeIndex into transparent reference type #9713

Job	Run time
Check that MkDocs can build the docs with Poetry 1.3.2	38s
build (ubuntu-latest)	3m 17s
Check that MkDocs can build the docs	26s
Check formatting	15s
build (macos-14)	2m 36s
build (windows-latest)	6m 42s
cargo-deny (advisories)	22s
build (ubuntu-latest, --all-features)	4m 17s
cargo-deny (bans licenses sources)	25s
Clippy check	2m 18s
Check protos	53s
	22m 9s

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