ES6 iterators - enable ForOf for collections

[vdolek]

This PR implements ES6 iterators for all collections in the library. It enables using ForOf with collections without using toArray() method.

Example:

const list = new List([1, 2, 3]);
for (const item of list) {
    console.log(item);
}

Besides that it allows using collections with *ngFor in Angular without using toArray() method.

[vdolek]

Hi @isc30, will you be able to review and possibly merge and publish this PR?

[isc30]

Hi there, this lib aims for ES5 compatibility and it has no Symbol.Iterator due to this limitation.
If you want, you can publish your fork in NPM as Linq-Collections-ES6 and get these new features rolling there!

[vdolek]

In the end this PR is published separately in https://www.npmjs.com/package/linq-collections-es6.

[isc30]

Awesome! Thanks a lot