RenderContext cleanup

[jrtom]

• a bunch of places assume that various Predicates or Functions can be null. Either use Optional or require them to be present instead. (Or mark as @nullable.)
• maybe don’t even have getBlahPredicate()/getBlahTransformer(); just expose isBlah()/applyBlah() instead
• edgeIncludePredicate() should just go ahead and check the edge’s endpoints as well so that the caller doesn’t have to
• edgeArrowPredicate/Transformer: just have something that provides an Optional Arrow for a given edge (get rid of the Predicate)
  • or a boolean that specifies for the entire RenderContext whether edge arrows are wanted
• figure out whether we want “? super V” or not
• use java.util.Function and .Predicate instead of their Guava counterparts
• consider making instance variables in PluggableRendererContext private instead of protected
• add @Override annotations
• add documentation for each method