Mark DS classes as final

[LeonMelis]

PECL's DS classes cannot be extended, as they are optimized C implementations.

Attempting to extend a DS class causes PHP to emit a fatal error:

PHP Fatal error:  Class MyVector cannot extend final class Ds\Vector

However, PHPStorm won't warn about this, because the stubs don't specify these classes as being final.

The official PECL DS polyfill (see: https://github.com/php-ds/polyfill/tree/master/src) does use the final keyword for the class definitions.

This PR adds the final keyword to the PECL DS class stubs, so that PHPStorm's code inspection correctly warns about attempting to extend a PECL DS class.