Generate an optional error if __init__ has no annotation but class has annotations

[JukkaL]

If __init__ has no annotation, it's likely that some attributes have implicit Any types, which seriously compromises type checking. This is a common mistake. Example where this is a problem:

class C:
    def __init__(self):
        self.a = []  # Inferred type for 'a' is Any, since no annotation

    def method(self) -> str:
        return self.a  # No error, since Any is compatible with str

Add an optional check to generate an error if __init__ has no type annotation but some attribute or method defined in the class body has a type annotation. The check would be enabled using --enable-error-code <code>.

Only look at definitions within the class body and ignore inherited __init__ and other inherited members.

This can be a little tricky to support in the daemon, since we'll need to add a dependency from every attribute to __init__ when the error code is enabled, if __init__ has no annotation (assuming this property is checked as part of checking __init__), and we need to keep track of whether each attribute has an annotation or not -- I'm not sure if the daemon keeps track of this or not.