Remote dealloc refactor. (#138)

Improve remote dealloc

- Outline the slow path to improve code gen significantly

- Handle message queue only on slow path for remote dealloc.

- Change remote size to count down 0, so fast path does not need a constant.

- Use signed value so that branch does not depend on addition.

src/mem/globalalloc.h

@@ -135,7 +135,7 @@ namespace snmalloc
 
           // Post all remotes, including forwarded ones. If any allocator posts,
           // repeat the loop.
-          if (alloc->remote.size > 0)
+          if (alloc->remote.capacity < REMOTE_CACHE)
           {
             alloc->stats().remote_post();
             alloc->remote.post(alloc->id());