Abstract iterable loop in built-in constructors to reduce down to one implementation that contains special case optimization for arrays

This is an idea I got from code reviewing #561.

There are numerous built-in constructors that optionally construct from a passed in iterable. In all of these constructors we could special case arrays so long as `Array.prototype[Symbol.iterator]` hasn't changed (or rather, the `[Symbol.iterator]` property on the given object is pointing to the array built-in value, and the object is itself an array).

It might be possible without complication to pull this common construct-from-iterable pattern out into a generic loop-over-iterable function that takes a lambda function for action on each element, and then implements the special case paths for arrays (and perhaps the other built-ins iterables Map, Set, and String).

This could also be applied to other loops over iterables in the runtime.