[strings] fixing and simplifying string tree iterator. This version is inspired by Python's itertools.product (itertoolsmodule.c has so many goodies)

src/string_utils.h

@@ -266,12 +266,8 @@ void string_tree_destroy(string_tree_t *self);
 
 typedef struct string_tree_iterator {
     string_tree_t *tree;
-    bool single_path;
     uint32_t *path;
-    uint32_t *num_alternatives;
     uint32_t num_tokens;
-    uint32_t cursor;
-    int8_t direction;           // 1 or -1
     uint32_t remaining;
 } string_tree_iterator_t;