Introduce `Box::new_uninit_array` and `Box::new_zeroed_array`

Currently the Box type has the methods Box::new_uninit_slice(len: usize) and Box::new_zeroed_slice(len: usize), which create Box<[MaybeUninit<T>]> with uninitialized and zeroed memory respectively.

I propose the creation of Box::new_uninit_array() and Box::new_zeroed_array(), which create Box<[MaybeUninit<T>; N]>, which create fixed-size arrays on the heap with a compile time size.

One example from my current use case (an I/O buffer):

struct Buffer<const N: usize> {
    data: Box<[MaybeUninit<u8>; N]>,
    ...
}

impl<const N: usize> Buffer<N> {
    pub fn new() -> Self {
        let data = Box::new_uninit_array();
        ....

        Self {
            data,
            ...
        }
    }
}

To my knowledge, the only way to do this currently is:

Box::new_uninit_slice(N).into_array().unwrap()

...which is both unstable and somewhat unsightly in my opinion with an unwrap from an unnecessary runtime check.

I would love to take a shot at making a PR myself, but I thought I should post here first before I did anything, I'm not sure of the exact process as this would be my first contribution to Rust. If anyone here has any objections or pointers please let me know.