Require new test to determine if reference variable.
All elements in containers are wrapped waldo objects.  That means that
instead of relying on python's native "in," we have to actually
iterate over list and compare internal values.
In addition to generating and incorporating deltas, switch to holding
references to containers meant that we also had to override the
references' de_waldoify method, which is used to change data back into
python to be passed to external code.
Code designed thinking it was operating on reference instead of on
internal list directly.