Touch-ups for dataclasses support

[tristanlatr]

• how the dataclasse (and classic attrs without the argument collect_by_mro=True) inherited attributes are ordered. It’s currently not quite right in case of colliding names.
• Support InitVar annotation.
• follow up issues should be opened to address (or at least document) the following missing features:
  • decorator based factory and converters feature in attrs
  • show the class decorators
  • make private (hidden by default) function and classes decorators that are private or inside a private namespace
• Verify that the default values of attrs/dataclasses fields when refers to a name are not considered as aliases
• take care of this comment: Handling of attrs and dataclass constructors #656 (comment)