Unsound usages of unsafe implementation from `()` to `Thread`

[llooFlashooll]

Hi, I am scanning the futures-timer in the latest version with my own static analyzer tool.

Unsafe conversion found at: src/native/global.rs#L86

fn raw_clone(ptr: *const ()) -> RawWaker {
    let me = ManuallyDrop::new(unsafe { Arc::from_raw(ptr as *const Thread) });
    mem::forget(me.clone());
    RawWaker::new(ptr, &VTABLE)
}

This unsound implementation would create a misalignment issues if the type size of () is smaller than the type size of Thread.

This would potentially cause undefined behaviors in Rust. If we further manipulate the problematic converted types, it would potentially lead to different consequences such as access out-of-bound. I am reporting this issue for your attention.