Proposal for faster this assignments in constructor functions

[dante federici]

Any reason we can't consider a scala-like constructor?

```js
class Point(x, y) {
  toString() {
    return `(${this.x},${this.y})`
  }
}

console.log(`??? ${new Point(1, 2)}`); // ??? 1,2
````

Doesn't conflict with existing syntax, it's clear, and you can still
shorthand most data-classes or structs to just:
```
class MyData(some, props, here) {}
```

I imagine this as the syntax:
```
class Point(x, y) { // implicits x, y as first two args of constru
  constructor(a, b, c) {
    this.z = a + b + c;
    console.log(this.x, this.y, this.c); // values of: a, b, c
    console.log(this.x === a, this.y === b); // true, true
  }
}
```

And de-sugared:
```
class Point {
  constructor(a, b, c) {
    this.x = a;
    this.y = b;
    // rest of body
    this.z = a + b + c;
  }
}
```
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