Modify heuristic for adding new slabs. (#429)

If there is only one slab remaining, then we probabalisticly allocator a
new one. If a slab is barely in use, then this could cause us to
effectively double the number of slabs in use.

This commit checks if the remaining slab has enough remaining elements
to provide randomisation.

src/ds/seqset.h

@@ -159,5 +159,13 @@ namespace snmalloc
         v.end = &(item->next);
       }
     }
+
+    /**
+     * Peek at next element in the set.
+     */
+    SNMALLOC_FAST_PATH const T* peek()
+    {
+      return v.head;
+    }
   };
 } // namespace snmalloc