Conditionally emit rest parameters

This is a sketch to omit code emission for the `...rest` parameter when it's not used in the body.

With this patch, the example in #498 will be compiled without generating code for the `...rest` parameter.

If this is the right approach, I can complete the implementation by handling the other cases in the new `isParameterReferencedInBody` method.

Might as well just let `forEachChild` take care of this.