/// Inserts a slice at a specific location in a vec.
pub fn insert_slice_at<T: Copy>(vec: &mut Vec<T>, index: usize, slice: &[T]) {
    unsafe {
        assert!(index <= vec.len());
        vec.reserve(slice.len());
        let insert_ptr = vec.as_mut_ptr().offset(index as isize);
        std::ptr::copy(
            insert_ptr,
            insert_ptr.offset(slice.len() as isize),
            vec.len() - index,
        );
        std::ptr::copy_nonoverlapping(slice.as_ptr(), insert_ptr, slice.len());
        vec.set_len(vec.len() + slice.len());
    }
}