Remote: store the sizeclass, too

Squeeze some bits out of allocator IDs so that we can land the sizeclass in
each Remote object.  The intent is that, on StrictProvenance architectures like
CHERI, we will be able to route Remote messages through RemoteCache-s without
needing to amplify back to read the sizeclass metadata field out of the slab
headers.

src/mem/globalalloc.h

@@ -126,7 +126,7 @@ namespace snmalloc
           if (alloc->remote.capacity < REMOTE_CACHE)
           {
             alloc->stats().remote_post();
-            alloc->remote.post(alloc->id());
+            alloc->remote.post(alloc->get_trunc_id());
             done = false;
           }