[ES6] Lift pure closures to outer scope

[Ginden]

Consider following code:

(function iife() {
    function foo(arr) {
        return arr.map(e=>e.someProperty).filter(Boolean)
    }
    function bar() {
          return arr.map(e=>e.someProperty).filter(e=>e===3)
    }
    return {foo, bar};
}())

This code can have e=>e.someProperty lifted to outer scope to avoid repeating code. The same optimization can be applied to constant expressions.

Pros:

• can result in smaller code size when repeating expression can't be optimized by gzip (especially in long files)
• can result in more memory-efficient code

Cons:

• (e=>e) !== (e=>e) - functions are objects and they are compared by identity, not by value
• can break code that relies on empty constructor with prototype methods.