std: Second pass stabilization for comm

This commit is a second pass stabilization for the `std::comm` module,
performing the following actions:

* The entire `std::comm` module was moved under `std::sync::mpsc`. This movement
  reflects that channels are just yet another synchronization primitive, and
  they don't necessarily deserve a special place outside of the other
  concurrency primitives that the standard library offers.
* The `send` and `recv` methods have all been removed.
* The `send_opt` and `recv_opt` methods have been renamed to `send` and `recv`.
  This means that all send/receive operations return a `Result` now indicating
  whether the operation was successful or not.
* The error type of `send` is now a `SendError` to implement a custom error
  message and allow for `unwrap()`. The error type contains an `into_inner`
  method to extract the value.
* The error type of `recv` is now `RecvError` for the same reasons as `send`.
* The `TryRecvError` and `TrySendError` types have had public reexports removed
  of their variants and the variant names have been tweaked with enum
  namespacing rules.
* The `Messages` iterator is renamed to `Iter`

This functionality is now all `#[stable]`:

* `Sender`
* `SyncSender`
* `Receiver`
* `std::sync::mpsc`
* `channel`
* `sync_channel`
* `Iter`
* `Sender::send`
* `Sender::clone`
* `SyncSender::send`
* `SyncSender::try_send`
* `SyncSender::clone`
* `Receiver::recv`
* `Receiver::try_recv`
* `Receiver::iter`
* `SendError`
* `RecvError`
* `TrySendError::{mod, Full, Disconnected}`
* `TryRecvError::{mod, Empty, Disconnected}`
* `SendError::into_inner`
* `TrySendError::into_inner`

This is a breaking change due to the modification of where this module is
located, as well as the changing of the semantics of `send` and `recv`. Most
programs just need to rename imports of `std::comm` to `std::sync::mpsc` and
add calls to `unwrap` after a send or a receive operation.

[breaking-change]

src/libstd/rand/os.rs

@@ -337,7 +337,7 @@ mod imp {
 mod test {
     use prelude::v1::*;
 
-    use comm::channel;
+    use sync::mpsc::channel;
     use rand::Rng;
     use super::OsRng;
     use thread::Thread;
@@ -363,7 +363,7 @@ mod test {
 
             Thread::spawn(move|| {
                 // wait until all the tasks are ready to go.
-                rx.recv();
+                rx.recv().unwrap();
 
                 // deschedule to attempt to interleave things as much
                 // as possible (XXX: is this a good test?)
@@ -384,7 +384,7 @@ mod test {
 
         // start all the tasks
         for tx in txs.iter() {
-            tx.send(())
+            tx.send(()).unwrap();
         }
     }
 }