Add unsize_box! macro to allow safe creation of Box<T: !Sized> on stable

[fu5ha]

Under stable feature, adds unsize_box! macro which takes as input a Box to a T: Sized and coerces it to a U: !Sized which can be safely unsize-coerced from T.

use allocator_api2::unsize_box;
use allocator_api2::boxed::Box;
use core::any::Any;

let sized_box: Box<u64> = Box::new(0);
let unsized_box: Box<dyn Any> = unsize_box!(sized_box);

See this discussion on the rust community discord for discussion, added a comment that explains it in the code as well https://discord.com/channels/273534239310479360/592856094527848449/1148216815844081734