NFC: FreeListIter domestication plumbing

Just an intermediate syntactic step to chase dependencies.  All these introduced
"domestication" callbacks are just the identity function, but they will let us
thread the LocalAlloc's handle to the Backend state down to where it's needed.

src/mem/localcache.h

@@ -82,6 +82,8 @@ namespace snmalloc
       typename SharedStateHandle::LocalState* local_state, DeallocFun dealloc)
     {
       auto& key = entropy.get_free_list_key();
+      auto domesticate = [](FreeObject::QueuePtr<capptr::bounds::Alloc> p)
+                           SNMALLOC_FAST_PATH_LAMBDA { return p; };
 
       for (size_t i = 0; i < NUM_SIZECLASSES; i++)
       {
@@ -89,7 +91,7 @@ namespace snmalloc
         // call.
         while (!small_fast_free_lists[i].empty())
         {
-          auto p = small_fast_free_lists[i].take(key);
+          auto p = small_fast_free_lists[i].take(key, domesticate);
           SNMALLOC_ASSERT(Metaslab::is_start_of_object(i, address_cast(p)));
           dealloc(p.as_void());
         }
@@ -99,18 +101,22 @@ namespace snmalloc
         local_state, remote_allocator->trunc_id(), key_global);
     }
 
-    template<ZeroMem zero_mem, typename SharedStateHandle, typename Slowpath>
-    SNMALLOC_FAST_PATH void* alloc(size_t size, Slowpath slowpath)
+    template<
+      ZeroMem zero_mem,
+      typename SharedStateHandle,
+      typename Slowpath,
+      typename Domesticator>
+    SNMALLOC_FAST_PATH void*
+    alloc(Domesticator domesticate, size_t size, Slowpath slowpath)
     {
       auto& key = entropy.get_free_list_key();
-
       sizeclass_t sizeclass = size_to_sizeclass(size);
       stats.alloc_request(size);
       stats.sizeclass_alloc(sizeclass);
       auto& fl = small_fast_free_lists[sizeclass];
       if (likely(!fl.empty()))
       {
-        auto p = fl.take(key);
+        auto p = fl.take(key, domesticate);
         return finish_alloc<zero_mem, SharedStateHandle>(p, sizeclass);
       }
       return slowpath(sizeclass, &fl);