stringid: add at method

stringid.hpp

@@ -24,6 +24,9 @@ namespace cruft {
         /// new ID if the string is not present.
         id_t operator[] (std::string_view const&);
 
+        // provided for symmetry with std member names
+        id_t at (std::string_view const &name) const { return find (name); }
+
 
         ///////////////////////////////////////////////////////////////////////
         id_t add  (std::string);