MvccRwLock allows data races & aliasing violations
Affected versions of the `noise_search` crate unconditionally implement Send/Sync for `MvccRwLock`.
This can lead to data races when types that are either `!Send` or `!Sync` (e.g. `Rc<T>`, `Arc<Cell<_>>`) are contained inside `MvccRwLock` and sent across thread boundaries. The data races can potentially lead to memory corruption (as demonstrated in the PoC from the original report issue).Also, safe APIs of `MvccRwLock` allow aliasing violations by allowing `&T` and `LockResult<MutexGuard<Box<T>>>` to co-exist in conflicting lifetime regions. The APIs of `MvccRwLock` should either be marked as `unsafe` or `MbccRwLock` should be changed to private or pub(crate).