avoid constructing TypeList's on-the-fly for ListVal's with fixed types

src/Val.h

@@ -661,8 +661,15 @@ private:
 class ListVal final : public Val
 	{
 public:
+	// Constructor used to build up a homogeneous list of values;
+	// or, if 't' is TYPE_ANY, then a heterogeneous one whose type
+	// is built up as values are appended.
 	explicit ListVal(TypeTag t);
 
+	// Constructor used to build the list in one shot, with the type
+	// pre-computed.
+	ListVal(TypeListPtr tl, std::vector<ValPtr> vals);
+
 	~ListVal() override = default;
 
 	TypeTag BaseTag() const { return tag; }