Add Mutex::arc method

[MarinPostma]

Mutexes are seldom used without an Arc, I propose to add a method Mutex::arc that returns an Arc<Mutex<_>>. This would make code more readable and compact almost everywhere mutexes are used. It also makes the intent clearer, as we first think about the Mutex, not the Arc when we use one.

// this:
let mutex = Arc::new(Mutex::new(Thing { inner: 0 }));
// would turn into this
let mutex = Mutex::arc(Thing { inner: 0 });

This would also mimic what we already have with the Box::pin method that return a Pin<Box<_>>.

[LeSeulArtichaut]

It would also probably make sense to add an arc method for RwLock?

[MarinPostma]

I am preparing a PR

[Stupremee]

SInce the original PR was closed, I will open a new PR

@rustbot claim

[JohnTitor]

I'm going to close this as #80007 was also closed. Thanks!