Warn on assignments of functions to classes

[KotlinIsland]

Needs some more thought, but:

Well

class A:
    a: Callable[[int], str] = lambda x: ""
A().a(1)  # no type error

So

from __future__ import annotations
class A:
    a: Callable[[int], str] = lambda x: ""  # E: A function on a class will become a method.  [function-breaks-callable]
                                            # N: Did you want "ClassVar[FunctionType]"/"Final[FunctionType]" instead?
    b: Callable[[A], str] = some_callable  # E: Some callable could be a function, externally verify it's not, or use "FunctionType"   [possible-function]
    c: FunctionType[[int], str] # ok

    def __init__(self):
        self.a = lambda x: ""
        self.c = lambda x: ""
A.a = lambda x: ""  # E: A function on a class will become a method.  [?]
                    # N: Did you want "FunctionType" instead?

• related to #617