- rewriting a runtimeCall of an instancemethod was broken (this is
  a separate code path from the much-more-common callattr form).
- we don't need to guard on the this->cls as often in Box::getattr, specifically
  in cases that are coming from typeLookup.  Not because the classes are fixed
  (I think they can change), but because they are not allowed to change in a way
  that would change what Box::getattr has the guard for (guarding on attrs_offset
  and tp_dictoffset).
- slightly change the place we guard on tp_getattro and tp_getattr to a place I think
  is a bit more correct (or at least easier to understand as being correct).