5.2.0: Closeable streams and files

• Made XmlStream and XmlFile implement the lang.Closeable interface
  (@thekid)
• Fixed Undefined property: util\address\XmlStream::$file warnings
  (@thekid)
• Changed XmlStream::toString() to check if InputStream implementations
  provide a toString() method even if they do not implement lang.Value.
  See xp-framework/core#310
  (@thekid)