Add pecl-ds 1.2.9

Data Structures (DS) is an extension providing specialized data structures as
efficient alternatives to the PHP array. You can read about it in more detail in
this blog post which highlights the API, performance and other benefits of using
the extension.

WWW: http://pecl.php.net/package/ds
WWW: https://github.com/php-ds/ext-ds

Reported by: Oliver Schonrock <oliver@schonrocks.com>