Issue a warning upon directly accessing an arena's bins

An arena's bins should normally be accessed via the `arena_get_bin`
function, which properly takes into account bin-shards. To ensure that
we don't accidentally commit code which incorrectly accesses the bins
directly, we mark the field with `__attribute__((deprecated))` with an
appropriate warning message, and suppress the warning in the few places
where directly accessing the bins is allowed.

include/jemalloc/internal/arena_structs.h

@@ -99,7 +99,9 @@ struct arena_s {
 	 * The arena is allocated alongside its bins; really this is a
 	 * dynamically sized array determined by the binshard settings.
 	 */
-	bin_t			bins[0];
+	JEMALLOC_WARN_ON_USAGE("Do not use this field directly. "
+	                       "Use `arena_get_bin` instead.")
+	bin_t			 all_bins[0];
 };
 
 #endif /* JEMALLOC_INTERNAL_ARENA_STRUCTS_H */