Test and fix Send and Sync traits of BTreeMap artefacts Fixes #76686. I'm not quite sure what all this implies. E.g. comparing with the definitions for `NodeRef` in node.rs, maybe an extra bound `T: 'a` is useful for something. The test compiles on stable/beta (apart from `drain_filter`) so I bet `Sync` is equally desirable. r? @Mark-Simulacrum