type analysis consolidation: instanceOfTpImplies
refactoring: prepares migration from Cond to Prop

doesn't change any behaviour
remove modelNull boolean in favor of subclassing
refactoring: drop Cond in favor of Prop

bringing the various analyses closer together

TODO: use SAT solver for implication checking in the CSE optimization
for now I'm checking implication using an ad-hoc mix
of hash-consing and equality/subset tests...

NOTE: prepareNewAnalysis resets said hash-consing, so must be
called before approximating a match to propositions
refactoring: make hash-consing more robust
refactoring: better name for symbolicCase