singleton: add a public query for validity

singleton.hpp

@@ -27,6 +27,13 @@ namespace cruft {
         static SelfT *instance;
 
     public:
+        static bool
+        valid (void) noexcept
+        {
+            return instance;
+        }
+
+
         /// instantiates the one blessed value of this type.
         ///
         /// `instantiate` must be called once and once only before any calls