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.

You mentioned earlier that I might want to make this function a bit more general - what do you have in mind?

I'm also thinking that it might be good to rename the function - 
`isParameterUsedInFunction(func: FunctionDeclaration, param: ParameterDeclaration): boolean` 
might be a better signature. Do you have any suggestions?