Warn about `Mutex<()>`

I can't think of a use case where this actually gives you any guarantees. You can still use it to protect another Sync data structure, but you can just as well forget to do so. One should always put the components that are protected into the Mutex even if not strictly necessary due to the componentes being thread safe themselves.

An example of such a situation would be

struct Foo {
    lock: Mutex<()>,
    a: AtomicUsize,
    b: AtomicUsize,
}

where one should not modify the two values one by one, but always together in a single critical section.

Use the issue body and its Mutex<()> example as the starting specification. First determine the intended warning scope and how it should handle protected Sync data structures and paired AtomicUsize values; done means the warning behavior and its false-positive boundaries are defined and verified.