std: Rename Chan/Port types and constructor

* Chan<T> => Sender<T>
* Port<T> => Receiver<T>
* Chan::new() => channel()
* constructor returns (Sender, Receiver) instead of (Receiver, Sender)
* local variables named `port` renamed to `rx`
* local variables named `chan` renamed to `tx`

Closes #11765

src/libstd/prelude.rs

@@ -61,7 +61,7 @@ pub use vec::{MutableVector, MutableTotalOrdVector};
 pub use vec::{Vector, VectorVector, CloneableVector, ImmutableVector};
 
 // Reexported runtime types
-pub use comm::{Port, Chan};
+pub use comm::{channel, Sender, Receiver};
 pub use task::spawn;
 
 // Reexported statics