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/freelist.h

@@ -416,12 +416,11 @@ namespace snmalloc
     /**
      * Moves the iterator on, and returns the current value.
      */
-    FreeObject::HeadPtr<BView, BQueue> take(const FreeListKey& key)
+    template<typename Domesticator>
+    FreeObject::HeadPtr<BView, BQueue>
+    take(const FreeListKey& key, Domesticator domesticate)
     {
       auto c = curr;
-      // TODO: Placeholder
-      auto domesticate = [](FreeObject::QueuePtr<BQueue> p)
-                           SNMALLOC_FAST_PATH_LAMBDA { return p; };
       auto next = curr->read_next(key, domesticate);
 
       Aal::prefetch(next.unsafe_ptr());