performance: Pre-emptively reserve size of the values vector

Each token will add a corresponding Variant into the vector, so we can
avoid the performance penalty connected with dynamic resizes of the
vector.

src/Eval.cpp

@@ -224,6 +224,8 @@ void Eval::evaluatePostfixStack (
 
   // This is stack used by the postfix evaluator.
   std::vector <Variant> values;
+  values.reserve(tokens.size());
+
   for (const auto& token : tokens)
   {
     // Unary operators.