Use a bitmap in extents_t to speed up search.

Rather than iteratively checking all sufficiently large heaps during
search, maintain and use a bitmap in order to skip empty heaps.

include/jemalloc/internal/extent_structs.h

@@ -102,6 +102,13 @@ struct extents_s {
 	 */
 	extent_heap_t		heaps[NPSIZES+1];
 
+	/*
+	 * Bitmap for which set bits correspond to non-empty heaps.
+	 *
+	 * Synchronization: mtx.
+	 */
+	bitmap_t		bitmap[BITMAP_GROUPS(NPSIZES+1)];
+
 	/*
 	 * LRU of all extents in heaps.
 	 *