SP: plumb CapPtr through cache_friendly functions

Change these functions from being void* to void* to either consuming or
producing FreeObject*-s as appropriate.

src/mem/freelist.h

@@ -141,6 +141,15 @@ namespace snmalloc
       return p.template as_static<FreeObject>();
     }
 
+    /**
+     * Construct a FreeObject for local slabs from a Remote message.
+     */
+    static CapPtr<FreeObject, CBArena> make(CapPtr<Remote, CBArena> p)
+    {
+      // TODO: Zero the difference between a FreeObject and a Remote
+      return p.template as_reinterpret<FreeObject>();
+    }
+
     /**
      * Read the next pointer handling any required decoding of the pointer
      */