lock_api's raw lock traits should support stateful guard markers

[zesterer]

Many possible implementations of Mutex, such as a ticket lock, require the guard type to hold state pertaining to the lock. Today, lock_api does not provide a way to store that state. A change to the API that solves this would look like:

pub unsafe trait RawMutex {
    type Guard;

    const INIT: Self;

    // Required methods
    fn lock(&self) -> Self::Guard;
    fn try_lock(&self) -> Option<Self::Guard>;
    // Takes `&mut Self::Guard` to comply with `Drop` requirements
    unsafe fn unlock(&self, guard: &mut Self::Guard);

    // Provided method
    fn is_locked(&self) -> bool { ... }
}

This could even be implemented in a semver-compatible way with a separate trait and a blanket impl like so:

pub unsafe trait RawMutex2 {
    type Guard;

    const INIT: Self;

    // Required methods
    fn lock(&self) -> Self::Guard;
    fn try_lock(&self) -> Option<Self::Guard>;
    // Takes `&mut Self::Guard` to comply with `Drop` requirements
    unsafe fn unlock(&self, guard: &mut Self::Guard);

    // Provided method
    fn is_locked(&self) -> bool { ... }
}

unsafe impl<M: RawMutex> RawMutex2 for M {
    type Guard = PhantomData<M::GuardMarker>;
    const INIT: Self = <M as RawMutex>::INIT;
    fn lock(&self) -> Self::Guard { <Self as RawMutex>::lock(self); PhantomData }
    fn try_lock(&self) -> Option<Self::Guard> { <Self as RawMutex>::try_lock(self).then_some(PhantomData) }
    unsafe fn unlock(&self, _: &mut Self::Guard) { <Self as RawMutex>::unlock(self); }
    fn is_locked(&self) -> bool { <Self as RawMutex>::is_locked(self) }
}

This also applies to the other raw traits in lock_api.

Edit: On reflection, a ticket lock is not the best example since it is possible to implement it without guard state. However, this is not true of its RwLock equivalent.