Replace Mutex<RefCell<T>> with RwLock<T>

[jonathanpallant]

An RwLock is a more gentle way to introduce both sharing and mutability in the same type.

[jonathanpallant]

I may have been confused. We have to do critical_section::Mutex<RefCell<T>> because the critical_section::Mutex is re-entrant. The std::sync::Mutex however gives you an object that implements DerefMut.