Incorporate szind/slab into rtree leaves.

Expand and restructure the rtree API such that all common operations can
be achieved with minimal work, regardless of whether the rtree leaf
fields are independent versus packed into a single atomic pointer.

include/jemalloc/internal/rtree_structs.h

@@ -2,11 +2,13 @@
 #define JEMALLOC_INTERNAL_RTREE_STRUCTS_H
 
 struct rtree_node_elm_s {
-	atomic_p_t	child;
+	atomic_p_t	child; /* (rtree_{node,leaf}_elm_t *) */
 };
 
 struct rtree_leaf_elm_s {
-	atomic_p_t	extent;
+	atomic_p_t	le_extent; /* (extent_t *) */
+	atomic_u_t	le_szind; /* (szind_t) */
+	atomic_b_t	le_slab; /* (bool) */
 };
 
 struct rtree_leaf_elm_witness_s {