alloc/arena: add inner allocator accessor

alloc/arena.hpp

@@ -128,6 +128,12 @@ namespace cruft::alloc {
         decltype(auto) reset (Args&&...args)
         { return m_store.reset (std::forward<Args> (args)...); }
 
+        /// Returns a const reference to the inner allocator.
+        ///
+        /// Do not return a non-const reference. If the user wants direct
+        /// access they should directly use a non-owning arena.
+        auto const& store (void) const& { return m_store; }
+
     private:
         AllocT m_store;
         arena<AllocT> m_arena;