concepts: add the null_stringy concept

concepts/string.hpp

@@ -13,11 +13,33 @@
 #include <string_view>
 
 namespace cruft::concepts {
+    /// A literal c-string character array.
+    template <typename T>
+    concept c_string =
+        std::is_array_v<std::remove_cvref_t<T>> and
+        std::is_same_v<
+            std::remove_cvref_t<
+                std::remove_all_extents_t<
+                    std::remove_cvref_t<T>
+                >
+            >,
+            char
+        >;
+
     /// A type that behaves like a string. ie, a sequence of characters
     template <typename T>
     concept stringy =
         std::same_as<T, std::string> or
         std::same_as<T, std::string_view> or
         std::same_as<T, char const*> or
-        std::same_as<T, char *>;
+        std::same_as<T, char *> or
+        c_string<T>;
+
+
+    /// A type that behaves like a string _and_ has a null terminator
+    template <typename T>
+    concept null_stringy =
+        std::same_as<T, std::string> or
+        std::same_as<T, char const*> or
+        std::same_as<T, char      *>;
 }