Prefer st_is_member over st_lookup with 0

The st_is_member DEFINE has simpler semantics, for more readable code.

array.c

@@ -5209,7 +5209,7 @@ flatten(VALUE ary, int level)
 	    }
 	    else {
 		id = (st_data_t)tmp;
-		if (st_lookup(memo, id, 0)) {
+		if (st_is_member(memo, id)) {
                     st_clear(memo);
 		    rb_raise(rb_eArgError, "tried to flatten recursive array");
 		}

class.c

@@ -1176,7 +1176,7 @@ method_entry_i(ID key, VALUE value, void *data)
 	if (!me) return ID_TABLE_CONTINUE;
 	if (!arg->recur && me->owner != owner) return ID_TABLE_CONTINUE;
     }
-    if (!st_lookup(arg->list, key, 0)) {
+    if (!st_is_member(arg->list, key)) {
 	if (UNDEFINED_METHOD_ENTRY_P(me)) {
 	    type = METHOD_VISI_UNDEF; /* none */
 	}

gc.c

@@ -3634,7 +3634,7 @@ cached_object_id(VALUE obj)
 
         while (1) {
             /* id is the object id */
-            if (st_lookup(objspace->id_to_obj_tbl, (st_data_t)id, 0)) {
+            if (st_is_member(objspace->id_to_obj_tbl, (st_data_t)id)) {
                 objspace->profile.object_id_collisions++;
                 id += sizeof(VALUE);
             }
@@ -7515,7 +7515,7 @@ gc_is_moveable_obj(rb_objspace_t *objspace, VALUE obj)
       case T_NODE:
       case T_CLASS:
         if (FL_TEST(obj, FL_FINALIZE)) {
-            if (st_lookup(finalizer_table, obj, 0)) {
+            if (st_is_member(finalizer_table, obj)) {
                 return FALSE;
             }
         }

parse.y

@@ -821,7 +821,7 @@ new_unique_key_hash(struct parser_params *p, VALUE ary, const YYLTYPE *loc)
 	    goto error;
 	}
 	if (!RB_TYPE_P(key, T_STRING)) goto error;
-	if (st_lookup(tbl, (st_data_t)RSTRING_PTR(key), 0)) goto error;
+	if (st_is_member(tbl, (st_data_t)RSTRING_PTR(key))) goto error;
 	st_insert(tbl, (st_data_t)RSTRING_PTR(key), (st_data_t)ary);
     }
     st_free_table(tbl);
@@ -11463,7 +11463,7 @@ error_duplicate_keys(struct parser_params *p, NODE *hash)
 	if (nd_type(head) != NODE_LIT) {
 	    yyerror1(&head->nd_loc, "key must be symbol literal");
 	}
-	if (st_lookup(literal_keys, (key = head->nd_lit), 0)) {
+	if (st_is_member(literal_keys, (key = head->nd_lit))) {
 	    yyerror1(&head->nd_loc, "duplicated key name");
 	}
 	else {

thread.c

@@ -3472,7 +3472,7 @@ rb_thread_key_p(VALUE self, VALUE key)
     if (!id || local_storage == NULL) {
 	return Qfalse;
     }
-    else if (st_lookup(local_storage, id, 0)) {
+    else if (st_is_member(local_storage, id)) {
 	return Qtrue;
     }
     else {

variable.c

@@ -1335,7 +1335,7 @@ rb_ivar_defined(VALUE obj, ID id)
 	break;
       case T_CLASS:
       case T_MODULE:
-	if (RCLASS_IV_TBL(obj) && st_lookup(RCLASS_IV_TBL(obj), (st_data_t)id, 0))
+	if (RCLASS_IV_TBL(obj) && st_is_member(RCLASS_IV_TBL(obj), (st_data_t)id))
 	    return Qtrue;
 	break;
       default: