Make the Iterator prototype have its own constructor

[nikolaybotev]

This change makes the generator prototype chain fully conformant with the spec the Iterator Helpers stage 3 proposal, and enables the following code to work when transpiled to older runtimes:

const Iterator = Object.getPrototypeOf(Object.getPrototypeOf(Object.getPrototypeOf((function* (){})()))).constructor;

// Add an iterator helper
Iterator.prototype.forEach = function() { /* ... */ }

// Make a custom Iterator class
class RepeatIterator extends Iterator { next() { return { value: 42 } } }

// The iterator helper is available here!
new RepeatIterator().forEach();

Of course, the above already works, but since Iterator.prototype currently returns the global Object prototype, installing forEach installs on all objects, which is not the intent, and RepeatIterator is not able to function as an Iterable, because it does not have the [Symbol.iterator] method. So, notably, with this PR, this will work as well:

for (const n of new RepeatIterator()) {
  console.log(n);
  break;
}
// prints 42

Also, note the above already works if regenerator is used in a standard configuration with preset-env in babel and core-js 3 is required (which polyfills the Iterator constructor, and regenerator picks that up as the "native" Iterator prototype), however, regenerator is already almost fully compliant with the prototype chain spec for generators, and I figured it is good for completeness to add this here, as maybe there are cases where regenerator is not used in that combination with the latest core-js that supplies a proper Iterator constructor implementation.

Unit tests have been added.

Addendum

I also went ahead and merged GeneratorFunctionPrototype into Generator, because the two were duplicates -- Generator is the prototype of GeneratorFunction! GeneratorFunctionPrototype was actually the one that was fully setup and Generator was not referenced anywhere in the prototype+constructor chain, except it had its own prototype property set , which allowed it to be used in an instanceof test. That test looks at Generator.prototype only, and because both Generator.prototype and GeneratorFunctionPrototype.prototype were set to the same GeneratorPrototype object, both Generator and GeneratorFunctionPrototype are interchangeable in this position!

In the graphic you can see GFP (GeneratorFunctionPrototype in the code) as the object properly setup in the prototype chain and Generator is crossed out. You can think of the change as a two-step - 1) remove Generator, 2) rename GeneratorFunctionPrototype to Generator.

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