std: Stabilize/deprecate features for 1.4

The FCP is coming to a close and 1.4 is coming out soon, so this brings in the
libs team decision for all library features this cycle.

Stabilized APIs:

* `<Box<str>>::into_string`
* `Arc::downgrade`
* `Arc::get_mut`
* `Arc::make_mut`
* `Arc::try_unwrap`
* `Box::from_raw`
* `Box::into_raw`
* `CStr::to_str`
* `CStr::to_string_lossy`
* `CString::from_raw`
* `CString::into_raw`
* `IntoRawFd::into_raw_fd`
* `IntoRawFd`
* `IntoRawHandle::into_raw_handle`
* `IntoRawHandle`
* `IntoRawSocket::into_raw_socket`
* `IntoRawSocket`
* `Rc::downgrade`
* `Rc::get_mut`
* `Rc::make_mut`
* `Rc::try_unwrap`
* `Result::expect`
* `String::into_boxed_slice`
* `TcpSocket::read_timeout`
* `TcpSocket::set_read_timeout`
* `TcpSocket::set_write_timeout`
* `TcpSocket::write_timeout`
* `UdpSocket::read_timeout`
* `UdpSocket::set_read_timeout`
* `UdpSocket::set_write_timeout`
* `UdpSocket::write_timeout`
* `Vec::append`
* `Vec::split_off`
* `VecDeque::append`
* `VecDeque::retain`
* `VecDeque::split_off`
* `rc::Weak::upgrade`
* `rc::Weak`
* `slice::Iter::as_slice`
* `slice::IterMut::into_slice`
* `str::CharIndices::as_str`
* `str::Chars::as_str`
* `str::split_at_mut`
* `str::split_at`
* `sync::Weak::upgrade`
* `sync::Weak`
* `thread::park_timeout`
* `thread::sleep`

Deprecated APIs

* `BTreeMap::with_b`
* `BTreeSet::with_b`
* `Option::as_mut_slice`
* `Option::as_slice`
* `Result::as_mut_slice`
* `Result::as_slice`
* `f32::from_str_radix`
* `f64::from_str_radix`

Closes #27277
Closes #27718
Closes #27736
Closes #27764
Closes #27765
Closes #27766
Closes #27767
Closes #27768
Closes #27769
Closes #27771
Closes #27773
Closes #27775
Closes #27776
Closes #27785
Closes #27792
Closes #27795
Closes #27797

src/libstd/primitive_docs.rs

@@ -111,8 +111,6 @@ mod prim_unit { }
 /// the raw pointer. It doesn't destroy `T` or deallocate any memory.
 ///
 /// ```
-/// #![feature(box_raw)]
-///
 /// let my_speed: Box<i32> = Box::new(88);
 /// let my_speed: *mut i32 = Box::into_raw(my_speed);
 ///