function writeString (stream, string) {
  var strlen = Buffer.byteLength(string)
  writeNumber(stream, strlen)

  stream.write(string, 'utf8')
}
writeString - write a utf8 string to the buffer

@param <Buffer> buffer - destination
@param <Number> pos - offset
@param <String> string - string to write
@return <Number> number of bytes written

@api private