lib: basic support for trees as sequences.

As of this commit, binary search trees can be iterated:
mapped over with mapcar and such.

* arith.c (poly, rpoly): rpoly won't work with trees. They
work just with vectors and lists so let's make the error
message more accurate. I noticed the self names of these two
are swapped; will fix in another commit.

* eval.c (tprint): Refactor to use iterator framework for
objects other than lists. The tree case falls into this,
so trees are supported.

* lib.h (enum seq_kind): New enum constant SEQ_TREELIKE.
(seq_iter_init_with_info): Declared.

* lib.c (seq_info): Map tree object to SEQ_TREELIKE type.
(seq_iter_get_tree, seq_iter_peek_tree): New static functions.
(seq_iter_rewind): Support rewinding tree iteration.
I think we could reuse the existing iterator here, and in the
hash case as well. I made a note to look into this.
(seq_iter_init_with_info): Internal linkage changed to
external, because tprint in eval.c uses this.
Handle SEQ_TREELIKE case here, by setting up iterator with the
two new static functions.
(seq_iter_mark): Handle SEQ_TREELIKE_CASE. Change switch
statement to exhaustively list cases.
(ldiff): Add SEQ_TREELIKE to various cases. Idea is that we
handle it like SEQ_HASH.
(nsort, sort, nshuffle, take, take_while, take_until,
drop_while, drop_until, update): Add case for SEQ_TREELIKE,
routing to error message.
(lazy_where_tree_func): New static function.
(where, sel, reject): Support trees.

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