standard library: 'max' and 'min' functions added (KT-3714, KT-3843, KT-3126)

We have two choices when min/max function is called for an empty collection: throw an exception (as Collections.min/max methods from JDK) or return null. I think the second choice is better. If you want to get an exception just add '!!' to the function call. But if you want to handle this case more gently you can use '?:' operator.