auto merge of #11598 : alexcrichton/rust/io-export, r=brson

* 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)

cc #11119

src/libsyntax/parse/mod.rs

@@ -298,7 +298,7 @@ mod test {
     use extra::serialize::Encodable;
     use extra;
     use std::io;
-    use std::io::mem::MemWriter;
+    use std::io::MemWriter;
     use std::str;
     use codemap::{Span, BytePos, Spanned};
     use opt_vec;