[mypyc] Optimize classmethod calls via cls

[JukkaL]

If the class has no subclasses, we can statically bind the call target:

class C:
    @classmethod
    def f(cls) -> int:
        return cls.g()  # This can be statically bound, same as C.g()

    @classmethod
    def g(cls) -> int:
        return 1

For this to be safe, also reject assignments to the "cls" argument in
classmethods in compiled code.

This makes the deltablue benchmark about 11% faster.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JukkaL]

Note that we can't easily use vtables to call class methods, since vtables are accessible through instances, not type objects.