Finalize @ShadowVariableLooped

[triceo]

• If we know the solution is invalid, why not terminate shadow var evaluation and set them all to null? Could help with performance. (ATM it looks as if we're already doing it.)
• There are arguments to be made in favor of updating the marker before the variable in question, so that the variable updater could use that information for something. (Unless we decide to ignore the variable in that case altogether.)
• If a problem cannot loop, make sure loop detection is completely disabled. (Case in point: VRP. VRP never needed loop detection.)
• Consider a different name for the annotation, based on user feedback. (ShadowVariablesConsistent?)