UnVarGraph: Improve asymptotics

This is a redesign of the UnVarGraph data structure used by the call
arity analysis to avoid the pathologically-poor performance observed in
issue #18789.  Specifically, deletions were previously O(n) in the case
of graphs consisting of many complete (bipartite) sub-graphs. Together
with the nature of call arity this would produce quadratic behavior.

We now encode deletions specifically, taking care to do some light
normalization of empty structures. In the case of the
`Network.AWS.EC2.Types.Sum` module from #19203, this brings the
runtime of the call-arity analysis from over 50 seconds down to less
than 2 seconds.

Metric Decrease:
    T15164
    WWRec