Array is already a SmartPtr, wrapping it in another is pointless.
Except it turns out that there are include dependencies preventing
us from using Array here. Use boost::intrusive_ptr to wrap an
ArrayData instead.
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.