Consolodate ArrayData position-based iterator methods

Implement these all in ArrayData, using iter_begin, advance,
rewind, and iter_end virtual methods.  Only SharedMap didn't
override the iter_* methods, so moved ArrayData's implementation
to SharedMap and tailored it a bit.  Fetch SharedMap's size at
construction time to avoid calls to SharedMap::vsize().
Removed HphpArray and NameValueTableWrapper implementations of
these methods.

hphp/runtime/base/array/array_data.cpp

@@ -307,8 +307,8 @@ void ArrayData::moveStrongIterators(ArrayData* dest, ArrayData* src) {
 }
 
 CVarRef ArrayData::endRef() {
-  if (size_t(m_pos) < size_t(size())) {
-    return getValueRef(size() - 1);
+  if (m_pos != invalid_index) {
+    return getValueRef(iter_end());
   }
   throw FatalErrorException("invalid ArrayData::m_pos");
 }