Tweak the interface of std::io

[alexcrichton]

• 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

[huonw]

make mem/buffered private modules

Does rustdoc handle this correctly? (I.e. will we still have MemWriter in the docs?)

[sfackler]

@huonw it does now. A private item that's publicly reexported once has its docs inlined at the reexport site.