cordyceps: Should `list::RawIter` impl Send?

Reported in https://github.com/tweedegolf/cfg-noodle/issues/14#issuecomment-2901241711, holding the iterator across an await point (e.g. using an async mutex for the `List`) means that the RawIter might need to impl send. IMO this is probably fine since we are bound by the lifetime of the mut borrow of the list.