concepts: Add Enum concept

Change-Id: Ib04b3ffde34cb0083f2038427095750bad828a27
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/stdplus/concepts.hpp b/include/stdplus/concepts.hpp
index ce020a6..374880c 100644
--- a/include/stdplus/concepts.hpp
+++ b/include/stdplus/concepts.hpp
@@ -25,4 +25,7 @@
 template <typename T>
 concept TriviallyCopyable = std::is_trivially_copyable_v<T>;
 
+template <typename T>
+concept Enum = std::is_enum_v<T>;
+
 } // namespace stdplus