Implement Stream automatically for Reader + Writer

This is consistent with the existing documentation but was not the
actual behaviour, which I've found to be rather a nuisance, actually.

Author: chris-morgan

src/libstd/rt/io/mod.rs

@@ -451,6 +451,8 @@ pub trait Writer {
 
 pub trait Stream: Reader + Writer { }
 
+impl<T: Reader + Writer> Stream for T;
+
 pub enum SeekStyle {
     /// Seek from the beginning of the stream
     SeekSet,