sexp_iterator_next, but independent of the previous value of the
iterator->type.
(sexp_iterator_first): Use sexp_iterator_parse.
(sexp_iterator_next): Likewise.
(sexp_iterator_enter_list): Use sexp_iterator_parse. SEXP_START
not needed anymore.
(sexp_iterator_exit_list): Likewise.

Rev: src/nettle/sexp.c:1.9