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

Implementing transpose() on Box<MaybeUninit<[T; N]>> did also cross my mind, not sure why I dismissed the idea to be honest. Sounds perfectly reasonable to me.

On another note, someone in the Rust Discord pointed me to this, which works, but it's unsafe, which is clearly not ideal.

let data = unsafe { Box::new_uninit().assume_init() };