Add Arc::from_thin_raw.

[conradludgate]

I have a setup where I want to do

let ptr = thin_arc.with_arc(Arc::clone).into_raw();

// store the raw pointer somewhere. then later

let thin_arc = ThinArc::from_data_raw(ptr);

I would go through Arc::from_raw(ptr).into_thin() but that doesn't work because from_raw needs T: Sized.

What I'm imagining is in this impl there is something like

    /// Reconstruct the Arc<HeaderSliceWithLength<H, [T]>> from a raw pointer obtained from into_raw()
    #[inline]
    pub unsafe fn from_thin_raw(ptr: *const HeaderSliceWithLength<H, [T]>) -> ThinArc<H, T> {
        let data_ptr = ptr as *const [u8] as *const u8;
        let thin_ptr = data_ptr.sub(offset_of!(ArcInner<HeaderSliceWithLength<H, [T; 0]>>, data));
        ThinArc {
            ptr: unsafe {
                ptr::NonNull::new_unchecked(
                    thin_ptr as *mut ArcInner<HeaderSliceWithLength<H, [T; 0]>>,
                )
            },
            phantom: PhantomData,
        }
    }

For context, I was trying to move some of the functionality from https://github.com/conradludgate/futures-buffered/blob/main/src/arc_slice.rs into triomphe.

The core idea is that I have ThinArc<H, Slot> where each Slot stores it's index in the slice.
Given a *const Slot, I can count back the index and get back the original ThinArc<H, Slot>. Almost everything was working, except for the drop() impl of the RawWaker, since I couldn't construct the arc with the correct size to deallocate.

[Manishearth]

That seems like a good idea