Export slug related functions

[c9s]

1. int r3_parse_slugs( char * path, char ** slugs )
   this function parses the slug names from the path.
   this function returns the length of the slugs. each slug is a null-terminated strings.

int list_len = r3_parse_slugs("/user/{id}", token_list);

1. r3_apply_slugs( char * path, ...)

   this function applies the variables into the slugs by the ordering. so an URL generator could be implemented.

        r3_apply_slugs("/user/{id}", 1, "123");
        r3_apply_slugs("/user/{id}/{title}", 2, "123", "title");