pickler: support classes that add __slots__ in a subclass

If the base class defines __slots__ and a derived class
also defines __slots__ then we were ignoring the ones defined
in the base class.

Walk the entire method-resolution-order class list when gathering
the slots to serialize.

Closes #72

Helped-by: Michael Scott Cuthbert <cuthbert@mit.edu>
Signed-off-by: David Aguilar <davvid@gmail.com>