Add nonfull_slabs to bin_stats_t.

When config_stats is enabled track the size of bin->slabs_nonfull in
the new nonfull_slabs counter in bin_stats_t. This metric should be
useful for establishing an upper ceiling on the savings possible by
meshing.

include/jemalloc/internal/bin_stats.h

@@ -45,6 +45,9 @@ struct bin_stats_s {
 	/* Current number of slabs in this bin. */
 	size_t		curslabs;
 
+	/* Current size of nonfull slabs heap in this bin. */
+	size_t		nonfull_slabs;
+
 	mutex_prof_data_t mutex_data;
 };