std: Stabilize APIs for the 1.12 release

std: Stabilize APIs for the 1.12 release

Stabilized

* `Cell::as_ptr`
* `RefCell::as_ptr`
* `IpAddr::is_{unspecified,loopback,multicast}`
* `Ipv6Addr::octets`
* `LinkedList::contains`
* `VecDeque::contains`
* `ExitStatusExt::from_raw` - both on Unix and Windows
* `Receiver::recv_timeout`
* `RecvTimeoutError`
* `BinaryHeap::peek_mut`
* `PeekMut`
* `iter::Product`
* `iter::Sum`
* `OccupiedEntry::remove_entry`
* `VacantEntry::into_key`

Deprecated

* `Cell::as_unsafe_cell`
* `RefCell::as_unsafe_cell`
* `OccupiedEntry::remove_pair`

Closes #27708
cc #27709
Closes #32313
Closes #32630
Closes #32713
Closes #34029
Closes #34392
Closes #34285
Closes #34529