Pattern matching

Rough draft:

```php
// Type checks, already possible today
$value is int;

// Literal checks
$value is 1;

// Omitting values
$value is [, $second];

// Deconstruction 
$value is Person(name: $name);
```

See also:

* https://github.com/iluuu1994/php-src/pull/102
* https://github.com/Kotlin/KEEP/pull/213
* https://openjdk.org/jeps/441 (including "Future work" section)