checkPackAllRevisions(): Create an object and store three different
revisions of the object.  Before packing, check to make sure all three
revisions can be loaded.  After packing, make sure all revisions are
gone (since they were not referenced by the root object, they are
garbage).

checkPackJustOldRevisions(): Same as above, but this time create a
root object and a link from the root object to the persistent object.
Then store three different revisions of the persistent object and make
sure all three revisions can be loaded.  After packing, make sure the
first two revisions are gone, but that the third revision (and the
root object) can be loaded.

Other changes:

- Add a ZERO global

- Add a Root class which does /not/ have a getoid() method (so its
  state will be pickled as normal, but attributes that point to
  `persistent' objects will be serialized as persistent ids).

- dumps(): In getpersid(), return None if the object has no getoid()
  method.  This ensures that the root object will get pickled
  correctly as described above.

- _newobj(), _makeloader(): Move these into a common base class called
  PackableStorageBase.  This base class also maintains the cache of
  persistent ids to Object instances, for unpickling.

- PackableStorage is the derived class that contains the actual tests.