Syntax sugar to assigning a constructor argument to an instance variable and to add the keyword this (or self)

Alternative 1, like Dart:
```pony
class Point
  var x: F64
  var y: F64

  new create(this.x, this.y)
```
Alternative 2, like Python, Typescript
```pony
class Point
  new create(var this.x: F64, var this.y: F64)
```