runtime: move r5js.ProcCall#bindResult into base class.
runtime: r5js.{Assignment,IdShim}: subclass r5js.ProcCallLike, not r5js.ProcCall.
runtime: make r5js.ProcCall.firstOperand private.
runtime: move r5js.ProcCall#clearEnv to base class and use from other subclasses.
runtime: begin decomposing r5js.IdShim#tryIdShim_.