Implement UpdateClass (#85)

Lets us write things like:
```py
type AttrsAsSets[T] = UpdateClass[
    *[Member[GetName[m], set[GetType[m]]] for m in Iter[Attrs[T]]]
]

class A:
    def __init_subclass__[T](
        cls: type[T],
    ) -> AttrsAsSets[T]:
        super().__init_subclass__()

class B(A):
    n: int
```

`B.n` will be interpretted as `set[int]`

Author: dnwpark