Optimize malloc() and free() fast paths.

Embed the bin index for small page runs into the chunk page map, in
order to omit [...] in the following dependent load sequence:
  ptr-->mapelm-->[run-->bin-->]bin_info

Move various non-critcal code out of the inlined function chain into
helper functions (tcache_event_hard(), arena_dalloc_small(), and
locking).

src/tsd.c

@@ -14,7 +14,7 @@ malloc_tsd_malloc(size_t size)
 {
 
 	/* Avoid choose_arena() in order to dodge bootstrapping issues. */
-	return arena_malloc(arenas[0], size, false, false);
+	return (arena_malloc(arenas[0], size, false, false));
 }
 
 void