Better handling of classes

[inumanag]

• Automatic ClassVar annotations for assignments with no type information
• Allow any statement within class body (as in Python) (Positional arguments not handled as in python3 #212)
• ClassVar inheritance (Inheritance oddness #208)
• Automatic self.slot deduction without specialized decorators (Class variable scoping doesn't match python3 #210)
• Class decorators (Unsupported class decorator #110)