Make std::rt::io::extensions public

This works around #9779, but is probably the right thing to do anyways
since that's the module where all of the documentation for those traits
lives.
auto merge of #9780 : sfackler/rust/extensions2, r=alexcrichton

This works around #9779, but is probably the right thing to do anyways
since that's the module where all of the documentation for those traits
lives.