m[set/map] Add operator bool()

Probably should use in places..

src/hb-map.hh

@@ -177,6 +177,7 @@ struct hb_hashmap_t
   }
 
   bool is_empty () const { return population == 0; }
+  explicit operator bool () const { return !is_empty (); }
 
   unsigned int get_population () const { return population; }
 

src/hb-set.hh

@@ -273,6 +273,7 @@ struct hb_set_t
 	return false;
     return true;
   }
+  explicit operator bool () const { return !is_empty (); }
 
   void dirty () { population = UINT_MAX; }