Rename std::io::AsRef{Reader,Writer} to std::io::ByRef{Reader,Writer}

cc @aturon. It seems a bit inconsistent that the new `std::io::AsRefReader` and `std::io::AsRefWriter` contain the single method `.by_ref()`. Instead we should rename the traits to `std::io::ByRefReader` and `std::io::ByRefWriter`. 
