isinstance(obj, cls) needs to do a bunch of dynamic checking: it needs to
check cls's class to see if it defines __instancecheck__, and it needs
to fetch __class__ on obj.  Most of those time those aren't overridden,
so __instancecheck__ gets skipped and __class__ returns the type of the object.

So use the same "type slot" machinery to cache whether an __instancecheck__
or custom __class__ attribute have gotten added.  These are a bit different
than the other slots since they are not "wrappers", they are simply bools
that say whether or not the attribute exists.  This makes the slot handling
code a bit messier / more divergent from CPython, but I think it still makes
sense to put this here since we get the hooking-on-attribute-updating and
update-all-subclasses-as-well automatically.