Add missing ShallowClone implementation for SetType

It turns out that SetType was missing a ShallowClone implementation.
Which meant that when a SetType was cloned, a TableType was received
(which has one less member). Which resulted in invalid memory accesses
when using the clone.

Thank you valgrind :)

src/Type.cc

@@ -484,6 +484,19 @@ SetType::SetType(TypeList* ind, ListExpr* arg_elements) : TableType(ind, 0)
 		}
 	}
 
+SetType* SetType::ShallowClone()
+	{
+	// constructor only consumes indices when elements
+	// is set
+	if ( elements && indices )
+		{
+		elements->Ref();
+		indices->Ref();
+		}
+
+	return new SetType(indices, elements);
+	}
+
 SetType::~SetType()
 	{
 	Unref(elements);

src/Type.h

@@ -381,6 +381,8 @@ public:
 	SetType(TypeList* ind, ListExpr* arg_elements);
 	~SetType() override;
 
+	SetType* ShallowClone() override;
+
 	ListExpr* SetElements() const	{ return elements; }
 
 protected: