boot+eval+platform+repl+test: pass Platform object through to r5js.boot.
platform+repl+test: remove a cyclic boot dependency.
eval+platform+repl: no need to pass scheme sources to worker.
platform+test: r5js.Platform+impls: remove fetchUrl(), add getTestSources().
repl: simplify r5js.repl.main.
platform+eval: move platform-specific r5js.Evaluator impls under platform/.
platform+test: split r5js.test.SchemeSources into test and non-test parts.