Use a queue of slabs for free lists

This commit changes the strategy for finding a free list from
a stack to a queue.  This tends to avoid the slow path considerably more.
It has some memory overheads.

TOOD:  We should move the bump allocation data out of the metaslab and
into the allocator.  At the moment, the slab contains the bump allocation
data, we should move this into the allocator, as it only ever has one slab
it is bump allocating from per sizeclass.

src/ds/dllist.h

@@ -63,6 +63,7 @@ namespace snmalloc
       std::is_same<decltype(T::next), T*>::value, "T->next must be a T*");
 
     T* head = Terminator();
+    T* tail = Terminator();
 
   public:
     bool is_empty()
@@ -96,6 +97,8 @@ namespace snmalloc
 
       if (head != Terminator())
         head->prev = item;
+      else
+        tail = item;
 
       head = item;
 #ifndef NDEBUG
@@ -103,6 +106,26 @@ namespace snmalloc
 #endif
     }
 
+    void insert_back(T* item)
+    {
+#ifndef NDEBUG
+      debug_check_not_contains(item);
+#endif
+
+      item->prev = tail;
+      item->next = Terminator();
+
+      if (tail != Terminator())
+        tail->next = item;
+      else
+        head = item;
+
+      tail = item;
+#ifndef NDEBUG
+      debug_check();
+#endif
+    }
+
     void remove(T* item)
     {
 #ifndef NDEBUG
@@ -111,6 +134,8 @@ namespace snmalloc
 
       if (item->next != Terminator())
         item->next->prev = item->prev;
+      else
+        tail = item->prev;
 
       if (item->prev != Terminator())
         item->prev->next = item->next;

src/mem/alloc.h

@@ -1073,7 +1073,7 @@ namespace snmalloc
         if ((allow_reserve == NoReserve) && (slab == nullptr))
           return nullptr;
 
-        sl.insert(slab->get_link());
+        sl.insert_back(slab->get_link());
       }
       auto& ffl = small_fast_free_lists[sizeclass];
       return slab->alloc<zero_mem>(

src/mem/slab.h

@@ -213,7 +213,7 @@ namespace snmalloc
         meta.link = index;
 
         // Push on the list of slabs for this sizeclass.
-        sl->insert(meta.get_link(this));
+        sl->insert_back(meta.get_link(this));
         meta.debug_slab_invariant(is_short(), this);
         return Superslab::NoSlabReturn;
       }