Delegate to imported implementation class

Works for methods, getters, and setters. Closes #7. Also does conversion on the values returned from the implementation, so that closes #3.

Note that the class's module file name is provided as the second parameter to the generator.
Stop saving Object.defineProperty to a variable

This was borne of some misguided attempt to be tamper-proof, but (a) we have a lot of work to do for that, if it's indeed a goal; (b) it doesn't even matter since we use Object.defineProperty immediately in the file, so there's no room for it to be tampered with in the meantime.