Rollup merge of #61244 - RalfJung:box, r=rkruppe

Box::into_vec: use Box::into_raw instead of mem::forget

`Box::into_raw` does, in one step, turn the `Box` into a raw ptr and avoid deallocation.  Seems cleaner than separating the two.

Also, `mem::forget` gets the `Box` with a `noalias` argument, but it is not actually correct that this is an exclusive pointer. So a stricter version of Stacked Borrows would complain here. (I can't actually make Stacked Borrows that strict yet though due to other issues.)