Simplifications and fixes

[jakobandersen]

A few simplifications and fixes of the code:

• Use Sphinx directives directly when possible (only derive to get an interface directive).
• Let Sphinx parse the base classes of class declarations.
• Simplify the anon declaration construction.
• Make a visual difference between struct and class directives.

[vermeeren]

Everything seems good to me and a few others I asked to check, thanks as usual!