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)
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