add is_unordered_set concept

bin/logger/main.cpp

@@ -1,4 +1,5 @@
 #include <set>
+#include <unordered_set>
 
 #include "hack/logger/logger.hpp"
 
@@ -9,6 +10,8 @@ auto main(int argc, char *argv[]) -> int
 
   std::vector<int> v { 1, 2, 3 };
   hack::log()(v, s);
+
+  std::unordered_set<int> us { 1, 2, 3 };
+
+  hack::log()(v, s, us);
 }
-
-