Eliminate some recursion from children of _expand_token_list

Previously, both _expand_node and _expand_function would always make
mutually recursive calls into _expand_token_list. This was unnecessary
since these functions can simply return unexpanded results, after which
the outer iteration will next attempt expansion of the results.

The only trick in doing this is to arrange so that the active list is
popped at the appropriate time. To do this, we add a new token_node_t
marker to the active stack. When pushing onto the active list, we set
marker to last->next, and when the marker is seen by the token list
iteration, we pop from the active stack.

glcpp.h

@@ -123,10 +123,16 @@ typedef struct skip_node {
 	struct skip_node *next;
 } skip_node_t;
 
+typedef struct active_list {
+	const char *identifier;
+	token_node_t *marker;
+	struct active_list *next;
+} active_list_t;
+
 struct glcpp_parser {
 	yyscan_t scanner;
 	struct hash_table *defines;
-	string_list_t *active;
+	active_list_t *active;
 	int lexing_if;
 	int space_tokens;
 	int newline_as_space;