iterator API: reject objects that don't make sense.

* lib.c (iter_more): Do not return t for unrecognized
objects, but thrown an exception. Do return t for
conses, which is necessary since they are iterators
for lists. Also, as a special case, we return t for
struct objects that don't have an iter-more method.
This is needed for the documented fast protocol.
Iterator objects implementing the fast protocol
still get iter-more invoked on them. The client
usually doesn't know that the iterator implements
the fast protocol, and so calls iter-more, which
unconditionally has to returns true.
(iter_item): Do not fall back on car(iter) for all
unhandled objects. Only conses are handled
via car. All unrecognized objects trigger an
exception.
(iter_step): Do not try to handle list-like
objects via cdr, only lists.
Improve the diagnostic for hitting the end of
an improper list: diagnostic shows the cons
cell rather than just the terminating atom.

* tests/012/iter.tl: Some test cases validating
that the functions error out for strings and
vectors. Much more coverage is possible here but
doesn't seem worth it; e.g. that the functions
reject a buffer, regex, function, ...

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