Change base to bases
Initial changes for multiple inheritance: base changed to bases,
but kept as list of symbols, not tuple of classes as it should be
to be able to implement type(name, bases, dict).
Adapted test_mro_disagreement.py to use class statement instead of
type(name, bases, dict).
Temporarily implemented type(obj) as a function, not an object.
Fixed error in ___fail() to match py-prelude.py