Add entity.FormatType()

entity/type.go

@@ -208,3 +208,13 @@ func TypesEqual (left, right Type) bool {
 	if  left == nil                    { return true  }
 	return left.Equals(right)
 }
+
+// FormatType returns a string representing a type. If the type is nil, it
+// returns "Void".
+func FormatType (ty Type) string {
+	if ty == nil {
+		return "Void"
+	} else {
+		return ty.String()
+	}
+}