Add static_assert IteratorBase size <= 16

ConflictSet.cpp

@@ -1944,6 +1944,8 @@ protected:
   // index into children array of particular leaf type
   int index;
 };
+// We want it to be cheap to copy
+static_assert(sizeof(IteratorBase) <= 16);
 
 struct IteratorBase::ChildAndMaxVersion {
   IteratorBase child;