- Only record structs when they are defined, not when they are declared
- If a struct was only declared when a typedef was created, it won't
  contain its member declarations. Those have to be drawn from the
  recorded structs instead.
- Rename internal members of AstParser to make it easier to debug

* Add Python 3.9 to CI
* Update AppVeyor image and always install all requirements
* Add Python 3.9 interop file
* Fix geninterop script and regenerate interop39.cs
    - Only record structs when they are defined, not when they are declared
    - If a struct was only declared when a typedef was created, it won't
      contain its member declarations. Those have to be drawn from the
      recorded structs instead.
   - Rename internal members of AstParser to make it easier to debug