add is_unordered_set concept

bin/concepts/main.cpp

@@ -1,4 +1,5 @@
 #include <set>
+#include <unordered_set>
 
 #include "hack/logger/logger.hpp"
 #include "hack/concepts/concepts.hpp"
@@ -15,6 +16,12 @@ void test_associative(const T& m)
   hack::log()("is associative", m);
 }
 
+template<hack::concepts::is_unordered_set T>
+void test_unordered_set(const T& m)
+{
+  hack::log()("is unordered set", m);
+}
+
 auto main(int argc, char *argv[]) -> int
 {
   std::map<std::string, int> m { { "a", 1 }, { "b", 2 } };
@@ -26,6 +33,10 @@ auto main(int argc, char *argv[]) -> int
 
   std::vector<int> v { 1, 2, 3 };
   // test_associative(v); error !!!
+
+  std::unordered_set<int> us { 1, 2, 3 };
+  test_unordered_set(us);
+  test_associative(us);
 }