Use Box::new_ununit etc. once that's stabilized

[koute]

Once the following methods are stabilized we should take advantage of them and clean up some of the unsafe code we've added to work around the fact that these are not stable (grep for the relevant TODOs in the code):

• Box::assume_init (tracking issue: Tracking issue for uninitialized constructors for Box, Rc, Arc rust-lang/rust#63291)
• Box::new_uninit
• Rc::new_uninit
• Arc::new_uninit
• MaybeUninit::slice_assume_init_mut (tracking issue: Tracking issue for #![feature(maybe_uninit_slice)] rust-lang/rust#63569)