io: add an optional displayConverter param to r5js.OutputSavingPort ctor.
test: add test cases for displaying values.
eval+test: turn r5js.EvaluateToExternalRepresentation interface => class.
eval+test: rename r5js.EvaluateToExternalRepresentation => r5js.EvalAdapter.
eval+test+misc: fold r5js.To{Js,String}Evaluator into r5js.EvalAdapter.