perf(hex8): use MKL Hessian products and exact zero pair addition

src/fesa/elements/hex8_high_low.cpp

@@ -14,7 +14,9 @@ using math_internal::HighLow;
 using Triple = std::array<HighLow, 3>;
 using Tensor = std::array<Triple, 3>;
 
-HighLow operator+(HighLow a, HighLow b) { return math_internal::Add(a, b); }
+HighLow operator+(HighLow a, HighLow b) {
+  return hex8_internal::AddPair(a, b);
+}
 HighLow operator-(HighLow a, HighLow b) {
   return math_internal::Subtract(a, b);
 }