Implement ES6 generators (yield / coroutine) in V4

• Simple introduction: https://x.st/javascript-coroutines/
• Fuller introduction: http://exploringjs.com/es6/ch_generators.html
• Spec: http://www.ecma-international.org/ecma-262/7.0/index.html#sec-generatorfunction-objects

I suspect this would involve some fairly serious re-engineering of the V4 engine ...