Stabilize BTree{Map,Set}::retain Closes #79025. FCP concluded here: https://github.com/rust-lang/rust/issues/79025#issuecomment-817201302 This is an approved feature on BTree{Map,Set} to mirror a functionality in Hash{Map,Set}, which has had some adequate testing since its introduction in https://github.com/rust-lang/rust/pull/79026 and doesn't seem to have caused any problems since.