One more py cleanup

Vecs are thin pointers over Exprs.
Vecs are used to track py-visible pointers to big data; when a Vec goes
dead then the py interpreter cannot "resurrect" a big data pointer.
Exprs holds the Big Data keys themselves dirctly; when they go dead the
Key can be deleted from the cluster.  Also, known-temp values (because
Vecs went dead) are flagged so complex expressions can optimize internal
temps