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/extent_externs.h

@@ -54,7 +54,7 @@ bool extent_purge_forced_wrapper(tsdn_t *tsdn, arena_t *arena,
     size_t length);
 extent_t *extent_split_wrapper(tsdn_t *tsdn, arena_t *arena,
     extent_hooks_t **r_extent_hooks, extent_t *extent, size_t size_a,
-    szind_t szind_a, size_t size_b, szind_t szind_b);
+    szind_t szind_a, bool slab_a, size_t size_b, szind_t szind_b, bool slab_b);
 bool extent_merge_wrapper(tsdn_t *tsdn, arena_t *arena,
     extent_hooks_t **r_extent_hooks, extent_t *a, extent_t *b);