Split up and standardize naming of stats code.

The arena-associated stats are now all prefixed with arena_stats_, and live in
their own file.  Likewise, malloc_bin_stats_t -> bin_stats_t, also in its own
file.

src/bin.c

@@ -29,7 +29,7 @@ bin_init(bin_t *bin) {
 	extent_heap_new(&bin->slabs_nonfull);
 	extent_list_init(&bin->slabs_full);
 	if (config_stats) {
-		memset(&bin->stats, 0, sizeof(malloc_bin_stats_t));
+		memset(&bin->stats, 0, sizeof(bin_stats_t));
 	}
 	return false;
 }