Apply stability attributes to std::comm

* channel() - #[unstable]. This will likely remain forever

* sync_channel(n: int) - #[unstable with comment]. Concerns have ben raised
  about the usage of the term "synchronous channel" because that generally only
  applies to the case where n == 0. If n > 0 then these channels are often
  referred to as buffered channels.

* Sender::send(), SyncSender::send(), Receiver::recv() - #[experimental]. These
  functions directly violate the general guideline of not providing a failing
  and non-failing variant. These functions were explicitly selected for being
  excused from this guideline, but recent discussions have cast doubt on that
  decision. These functions are #[experimental] for now until a decision is made
  as they are candidates for removal.

* Sender::send_opt(), SyncSender::send_opt(), Receiver::recv_opt() - #[unstable
  with a comment]. If the above no-`_opt` functions are removed, these functions
  will be renamed to the non-`_opt` variants.

* SyncSender::try_send(), Receiver::try_recv() - #[unstable with a comment].
  These return types of these functions to not follow general conventions. They
  are consistent with the rest of the api, but not with the rest of the
  libraries. Until their return types are nailed down, these functions are
  #[unstable].

* Receiver::iter() - #[unstable]. This will likely remain forever.

* std::com::select - #[experimental]. The functionality is likely to remain in
  some form forever, but it is highly unlikely to remain in its current form. It
  is unknown how much breakage this will cause if and when the api is
  redesigned, so the entire module and its components are all experimental.

* DuplexStream - #[deprecated]. This type is not composable with other channels
  in terms of selection or other expected locations. It can also not be used
  with ChanWriter and ChanReader, for example. Due to it being only lightly
  used, and easily replaced with two channels, this type is being deprecated and
  slated for removal.

* Clone for {,Sync}Sender - #[unstable]. This will likely remain forever.