main: r5js.IPipeline#Eval: remove onOutput param.
main: r5js.PublicApi#Eval: remove sideEffectHandler param.
main: add r5js.IPipeline#desugarRepl as temporary shim.
main: rename r5js.PublicApi#Eval => evaluate.
main: add and use r5js.Evaluator interface abstracting r5js.PublicApi.
main: rename r5js.PublicApi => r5js.EvaluatorImpl.
eval: move evaluator_impl.js under eval/.
eval: run evaluator_impl.js through the linter.
eval: correct JsDoc of r5js.EvaluatorImpl#evaluate.