Tweak the interface of std::io

* Reexport io::mem and io::buffered structs directly under io, make mem/buffered
  private modules
* Remove with_mem_writer
* Remove DEFAULT_CAPACITY and use DEFAULT_BUF_SIZE (in io::buffered)

src/libsyntax/print/pprust.rs

@@ -32,7 +32,7 @@ use std::cell::RefCell;
 use std::char;
 use std::str;
 use std::io;
-use std::io::mem::MemWriter;
+use std::io::MemWriter;
 
 // The &mut State is stored here to prevent recursive type.
 pub enum AnnNode<'a,'b> {