Add read_to_str and write_{str, line}

These methods are sorely needed on readers and writers, and I believe that the
encoding story should be solved with composition. This commit adds back the
missed functions when reading/writing strings onto generic Readers/Writers.
auto merge of #10861 : alexcrichton/rust/iostr, r=pcwalton

These methods are sorely needed on readers and writers, and I believe that the
encoding story should be solved with composition. This commit adds back the
missed functions when reading/writing strings onto generic Readers/Writers.