[Class] update class.rbs

[sampersand]

This closes soutaro/steep#2124 by updating class.rbs. Few important things to note that I'd like input on:

1. I've added a [self: Class] to Class#initialize—it seems correct, but if we should remove that LMK

And other things to note:

1. Technically Class defines both a singleton Class.allocate and an instance Class#allocate. I've gone ahead and included both for completeness, and it shouldn't matter
2. inherited is now private (as it should be) and returns void (as nothing in Ruby uses its return value)
3. new's signature now accepts anything
4. attached_object still returns untyped

[ParadoxV5]

2. attached_object now returns an instance instead of untyped. Is this correct?

On second thought: It’s not correct.
It should be instance.instance (instance²), or T where self < singleton(T) (see #1542).

[sampersand]

Yeah, i've just removed it entirely and it's now just untyped again