Replace all 'Array.forEach' with 'for of' cycle

[IvanGoncharov]

Is there a concrete reason to make this change? .forEach is how most things are done in this library, and I'd prefer to maintain consistency.

@mjmahone for of is 4x faster than forEach + arrow function: https://jsperf.com/for-of-vs-foreach5
If we enable loose mode for for of than it will be 12x speedup.
Also, for of don't force us to create closures so it should lower memory usage in recursive functions.

But more importantly, the main purpose of this library is to be the reference implementation of GraphQL so code readability is very important.

[mjmahone]

I personally disagree with for of being more readable, but I come from a very functional-language background. The performance benefit though is tangible and significant enough that I'm on board!