make Nitpick's handling of definitions more robust in the face of formulas that don't have the expected format (needed for soundness, cf. RNG100+1)

fixed soundness bug in Nitpick related to unfolding -- the unfolding criterion must at least as strict when looking at a definitional axiom as elsewhere, otherwise we end up unfolding a constant's definition in its own definition, yielding a trivial equality

more robust Swing_Thread.now: allow body to fail;