We only have recursion if what we're currently looking at is the same as
one of its parents in the nested arrays. We don't need to keep track of
everything that's been seen, only the elements seen in a path down to that
element.
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.
aalexandre