op: Use std::vector's insert member function within vector variant of Add()

While looping here does work fine, it's mildly inefficient, particularly
if the number of members being added is large, because it can result in
multiple allocations over the period of the insertion, depending on how
much extra memory push_back may allocate for successive elements.

Instead, we can just tell the std::vector that we want to slap the whole
contained sequence at the back of it with insert, which lets it allocate
the whole memory block in one attempt.

src/op.cpp

@@ -115,9 +115,7 @@ void Op::Add(std::string string) {
 }
 
 void Op::Add(const std::vector<Id>& ids) {
-    for (Id op : ids) {
-        Add(op);
-    }
+    operands.insert(operands.end(), ids.begin(), ids.end());
 }
 
 u16 Op::WordCount() const {