It was discovered that concrete nodes tend to get way
too many groups in many cases, and the cause of this
seems to be table aliasing in underride.

Underride no longer reuses (aliases) the original source
tables when it copies a table into the target, but makes
a deep copy.  This is a significant change because the
underride logic is used in many places, and so this has
very broad impact.  This may fix other bugs caused by
table aliasing, including ones that may be "load-bearing",
so we will need to be on the look-out for any unexpected
changes in behavior.

In order to work for underride, deepcopy needed to be
extended to support cycles and other non-simple graph
structures.

This may have some performance impact where
underride is used (mostly startup time), and where
deepcopy is used (DNTs) but that should not be very
large as long as data structures are not excessively
complex.