Support callable classes

[felix-hilden]

Partially from #96. We should support linking callable classes. For example:

class A:
    b: int = 3

    def __call__(self) -> "A": return self

def foo() -> A: return A()

# example:
a = A()
a().b
foo()().b
A()()()()()()()()()()()()()()()().b

[felix-hilden]

We'll have to change our resolving logic, because currently there is no distinction between a type and its instance. This leads to call chains repeatedly "initialising" (=no-op) the class without actually considering the __call__ method.