Remove the extension traits for Readers/Writers

These methods are all excellent candidates for default methods, so there's no
need to require extra imports of various traits.

src/libsyntax/ext/source_util.rs

@@ -20,7 +20,7 @@ use parse::token::{get_ident_interner};
 use print::pprust;
 
 use std::rt::io;
-use std::rt::io::extensions::ReaderUtil;
+use std::rt::io::Reader;
 use std::rt::io::file::FileInfo;
 use std::str;