Leak &mut [T] in favor of NonNull<[T]>

```rust use leak_slice::LeakSliceExt; use std::mem::forget;

fn main() { let slice = &mut [1, 3, 3, 7][..]; let ptr = slice.leak();

// forget about the slice
forget(slice); // optional, but still don't use slice

// SAFETY: we forgot about the slice
unsafe {
    assert_eq!(ptr.as_ref(), [1, 3, 3, 7]);
}

} ```