dumps(), makeloader(): Take these out of the PackableStorage class,
and keep them even though they aren't currently used.

checkOldRevisionPacked(): Removed -- it's too complicated as a first
test.

checkSimplePack(): New pack() checker; it creates three revisions of
the same object, then attempts to pack to `now'.  Then it makes sure
that revisions 1 and 2 of the object are gone, but that revision 3
(the current revision) still exists.  Full passes this test, but
FileStorage fails. :(