Bottom commits from BatchIt (#675)

* msvc: set __cplusplus to the actual value in use

* ds_core/bits: add mask_bits; convert one_at_bit-s

* remotecache: enable reserve_space multiple objects

* nits

* Small changes to tracing

- Trace "Handling remote" once per batch, rather than per element

- Remote queue events also log the associated metaslab; we'll use this
  to assess the efficacy of https://github.com/microsoft/snmalloc/issues/634

* freelist builder: allow forcibly tracking length

* Try forward declaring freelist::Builder to appease macos-14

* freelist: tweak intra-slab obfuscation keys by meta address

* NFC: freelist: allow `next` to be arbitrary value

* Switch to a central, tweaked key for all free lists

* allocconfig: introduce some properties of slabs

We'll use these to pack values in message queues.

- Maximum distance between two objects in a single slab
- Maximum number of objects in a slab

* NFC: Templatize LocalCache on Config

* NFC: split dealloc_local_object_slow

We'll use the _slower form when we're just stepping a slab through
multiple rounds of state transition (to come), which can't involve
the actual memory object in question.

* NFC: make freelist::Object::T-s by placement new

* NFC: CoreAlloc: split dealloc_local_object

The pattern of `if (!fast()) { slow() }` occurs in a few places, including in
contexts where we already know the entry and so don't need to look it up.

src/test/func/domestication/domestication.cc

@@ -137,7 +137,8 @@ int main()
 
   LocalEntropy entropy;
   entropy.init<DefaultPal>();
-  RemoteAllocator::key_global = FreeListKey(entropy.get_free_list_key());
+  entropy.make_free_list_key(RemoteAllocator::key_global);
+  entropy.make_free_list_key(freelist::Object::key_root);
 
   auto alloc1 = new Alloc();