(sexp_iterator_assoc): Don't enter the list, associate

keys within the current list. Still exit the list when done.
(sexp_iterator_assoc): Represent keys as plain NUL-terminated
strings.
(sexp_iterator_check_type, sexp_iterator_check_types): New
functions.

Rev: src/nettle/sexp.c:1.4
Rev: src/nettle/sexp.h:1.3