Refactor: Handler instances self param

This pull request refactors the API to pass the value the method is invoked on as first argument to handlers' methods instead of filling in `$this` with it. This way, we're able to overcome the limitation of not being able to modify the value in PHP >= 5.6.0 (making the `array_pop` example work once again) and gets rid of segmentation faults when PHP relies on `$this` being an object.

## Old
```php
class StringHandler {
    public function length() {
        return strlen($this);
    }
}
```

## New 
```php
class StringHandler {
    public function length($self) {
        return strlen($self);
    }
}
```