Consider renaming Mutex<T> -> MutexCell<T>

[carllerche]

This would mirror RefCell / UnsafeCell / Cell and allow non-cell mutexes to be used easier in the same module.

[reem]

If the goal is to make raw::Mutex easier to use, than renaming it to RawMutex or using it as raw::Mutex instead of fully importing it might be better solutions. sync::Mutex is used much more often than raw::Mutex, and I would not want to make its name longer.

[huonw]

cc #14530, #15724

[steveklabnik]

/cc @aturon

[reem]

fwiw I've changed my mind here, I'm not totally for the renaming but I withdraw my objection.

[alexcrichton]

With the stabilization of these types I do not believe we are going to do this renaming.