set/dset: add `insert` for symmetry with the STL

set/dset.hpp

@@ -79,6 +79,11 @@ namespace cruft::set {
             return add (1, val);
         }
 
+        void insert (ValueT const &val)
+        {
+            return add (val);
+        }
+
         /// A list of items to remove from this store.
         ///
         /// Items must be in sorted order, and must be a subset of this