Name-based + new position-based destructuring

This is an issue to discuss **name-based destructuring** and **new syntax for position-based destructuring**. The current full text of the proposal can be found [here](https://github.com/Kotlin/KEEP/blob/name-based-destructuring/proposals/name-based-destructuring.md). We propose a new form of _destructuring_ based on _names_ instead of positions. In addition, we propose new syntax for _position_-based destructuring. ```kotlin (val age, val name = fullName) = person ```