Change null handling when creating Ids

[madsmtm]

Option::unwrap_unchecked is stable since 1.58, and that is IMO much clearer than wrapping things in NonNull beforehand (see the changelog entry for an example).

The real motivation for this change is from #81, where the naive implementation of Id::retain_autoreleased_null would have been completely broken (the extra branch in NonNull::new(ptr).map(|ptr| unsafe { Id::retain_autoreleased(ptr) }) would interfere).

Also, looking through the code I seem to have used NonNull::new_unchecked a lot more than what's entirely correct (+ alloc/+ new can return nil when out of memory), so changing those to unwrap makes things better in that regard (albeit a tiny bit slower).