change ngx_http_variable_value_node_t to more generic ngx_str_node_t

src/core/ngx_string.h

diff --git a/src/core/ngx_string.h b/src/core/ngx_string.h
index 53b9422..86a9601 100644
--- a/src/core/ngx_string.h
+++ b/src/core/ngx_string.h
@@ -200,6 +200,17 @@
 uintptr_t ngx_escape_html(u_char *dst, u_char *src, size_t size);
 
 
+typedef struct {
+    ngx_rbtree_node_t         node;
+    ngx_str_t                 str;
+} ngx_str_node_t;
+
+
+void ngx_str_rbtree_insert_value(ngx_rbtree_node_t *temp,
+    ngx_rbtree_node_t *node, ngx_rbtree_node_t *sentinel);
+ngx_str_node_t *ngx_str_rbtree_lookup(ngx_rbtree_t *rbtree, ngx_str_t *name,
+    uint32_t hash);
+
 
 void ngx_sort(void *base, size_t n, size_t size,
     ngx_int_t (*cmp)(const void *, const void *));