Fix some RefData<->TypedValue<->Variant<->Value type puns

Our ext_hhvm generated code is casting TypedValue* to Value*
on the assumption that the offset of TypedValue::m_data is 0.
Fix this assumption, and also while in the same code, replace
some (t == KindOfString || t == KindOfStaticString) with
IS_STATIC_STRING(t), which does a single bit test instead of
two comparisons.

hphp/runtime/base/array/array_iterator.cpp

@@ -550,7 +550,7 @@ MArrayIter::~MArrayIter() {
   }
   // unprotect the data
   if (hasVar()) {
-    RefData* ref = (RefData*)getVar();
+    RefData* ref = RefData::refDataFromVariantIfYouDare(getVar());
     decRefRef(ref);
   } else if (hasAd()) {
     decRefArr(getAd());