code: func NewStdWriter(w io.Writer, t StdType) io.Writer {
  	return &stdWriter{
  		Writer: w,
  		prefix: byte(t),
  	}
  }
docs: // NewStdWriter instantiates a new Writer.
  // Everything written to it will be encapsulated using a custom format,
  // and written to the underlying `w` stream.
  // This allows multiple write streams (e.g. stdout and stderr) to be muxed into a single connection.
  // `t` indicates the id of the stream to encapsulate.
  // It can be stdcopy.Stdin, stdcopy.Stdout, stdcopy.Stderr.