abstract and read only attributes

[KotlinIsland]

• resolves error on calls to abstract class methods #752
• resolves invent a way to make abstract methods work with mro #721
• resolves any class with ABC should be abstract #714
• resolves support abstract attributes/class attributes #709
• resolves show error on class that doesn't implement abstract methods #696
• resolves Abstract/abstract annotation/decorator #364

[ghost]

what about:

@abstract
class A:
    @abstract
    def f(self): ...

class B:
    def f(self):
        print("hi")

class C(A, B):  # error, this is abstract
    pass