Pimple needs to implement ``IndexAccess<string, mixed>`` in its HAck version, to support using Pimple as ``$pimple['foo']``. Otherwise, we loose a lot compared to the PHP version.
See http://docs.hhvm.com/manual/en/class.hack.arrayaccesstktv.php (``ArrayAccess`` in Hack code is here for compatibility with PHP code, but it is not a first-class citizen for the Hack code)