- SLOT1BINFULL() macro: changed this to check for __rop__ overriding
  __op__, like binary_op1() in abstract.c -- the latter only calls the
  slot function once if both types use the same slot function, so the
  slot function must make both calls -- which it already did for the
  __op__, __rop__ order, but not yet for the __rop__, __op__ order
  when B.__class__ is a subclass of A.__class__.

- slot_sq_contains(), slot_nb_nonzero(): use lookup_maybe() rather
  than lookup_method() which sets an exception which we then clear.

- slot_nb_coerce(): don't give up when left argument's __coerce__
returns NotImplemented, but give the right argument a chance.