`Borrow` & `make_mut`

I tried to integrate `rclite` in a project of mine ([jaq](https://github.com/01mf02/jaq)), but I can't because jaq needs `Rc::make_mut` and `Borrow`. Could you implement these? If so, then I could integrate `rclite` into jaq, providing you with some real-life performance benchmarks of `alloc::rc::Rc` vs `rclite::Rc`. :)