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/localalloc.h

@@ -211,6 +211,9 @@ namespace snmalloc
     SNMALLOC_FAST_PATH void* small_alloc(size_t size)
     {
       //      SNMALLOC_ASSUME(size <= sizeclass_to_size(NUM_SIZECLASSES));
+
+      auto domesticate = [](FreeObject::QueuePtr<capptr::bounds::Alloc> p)
+                           SNMALLOC_FAST_PATH_LAMBDA { return p; };
       auto slowpath =
         [&](
           sizeclass_t sizeclass,
@@ -239,7 +242,7 @@ namespace snmalloc
           };
 
       return local_cache.template alloc<zero_mem, SharedStateHandle>(
-        size, slowpath);
+        domesticate, size, slowpath);
     }
 
     /**