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/thread_local/mod.rs

@@ -471,7 +471,7 @@ mod imp {
 mod tests {
     use prelude::v1::*;
 
-    use comm::{channel, Sender};
+    use sync::mpsc::{channel, Sender};
     use cell::UnsafeCell;
     use thread::Thread;
 
@@ -480,7 +480,7 @@ mod tests {
     impl Drop for Foo {
         fn drop(&mut self) {
             let Foo(ref s) = *self;
-            s.send(());
+            s.send(()).unwrap();
         }
     }
 
@@ -497,9 +497,9 @@ mod tests {
             FOO.with(|f| unsafe {
                 assert_eq!(*f.get(), 1);
             });
-            tx.send(());
+            tx.send(()).unwrap();
         });
-        rx.recv();
+        rx.recv().unwrap();
 
         FOO.with(|f| unsafe {
             assert_eq!(*f.get(), 2);
@@ -519,7 +519,7 @@ mod tests {
                 *f.get() = Some(Foo(tx.take().unwrap()));
             });
         });
-        rx.recv();
+        rx.recv().unwrap();
     }
 
     #[test]
@@ -610,7 +610,7 @@ mod tests {
             let mut tx = Some(tx);
             K1.with(|s| *s.get() = Some(S1(tx.take().unwrap())));
         });
-        rx.recv();
+        rx.recv().unwrap();
     }
 }