Make it work on stable rust 1.53

[pudnax]

Feature const_in_array_repeat_expressions have been removed due to ambiguous drop order of ?Sised objects and other whatever reasons link.

And workaround is to make a constant for the value:

const SAMPLE: Option<F> = None;
pub fn new() -> Self {
    Executor {
        task_queue: [Self::SAMPLE; N],
    }
}

And below the using 0 as *const () is probably safe, but better to use ptr::null() and clippy also has style lint for this situation zero_ptr

Value of NULL is guaranteed by the standard to be 0. In practice, this particular feature was used in the first PDP machines, if I am not mistaken. if (ptr) and if (ptr == 0) and if (ptr == NULL) are always and everywhere equivalent https://twitter.com/typesanitizer/status/1333171905872891904

p.s.: I'm good at bad names :x

[XiangpengHao]

Looks good to me, thank you for the fix!