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