{get,set}NthKid() should be only used to walk thru all the kids and not to get/set a known specific kid. Replace its use by direct helper method.
All remaining callers of {get,set}NthKid() were audited and there are no instances left accessing specific kid.
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.
jan