Have constructor return a reference to the value_type (#39)

* Update `constructor` so that the call operator returns a pointer to the slot.

* Return a refernce to the `value_type`

Author: greg7mdp

include/gtl/phmap.hpp

@@ -2143,14 +2143,14 @@ class raw_hash_set {
             return *slot_;
         }
 
-        // returns a pointer to the inserted value.
-        // beware that the returned pointer is stable only for `node` hash map or sets. When using
-        // a `flat` version, this pointer can be used immediately, but shouldn't be stored.
+        // returns a reference to the inserted `value_type`.
+        // beware that the returned reference is stable only for `node` hash map or sets. When using
+        // a `flat` version, this reference can be used immediately, but we shouldn't store a pointer to it.
         template<class... Args>
-        slot_type* operator()(Args&&... args) const {
+        const auto& operator()(Args&&... args) const {
             assert(*slot_);
             PolicyTraits::construct(alloc_, *slot_, std::forward<Args>(args)...);
-            return *slot_;
+            return PolicyTraits::element(slot());
         }
 
     private: