[m-rev.] for review: replace the item list with structured ASTs

[Zoltan Somogyi]

For review by Julien (thanks for volunteering) and anyone else
who is interested. It passes all test cases, except a newly added one,
for which the reason is noted in the log.

The diff is too big to be posted at once, so the following message
will contain its second part.

This diff leaves a lot of further improvements as-yet unimplemented,
in order to make this diff easier to finish, and to review. I would also
appreciate opinions on the preferred order of making those further
improvements.

Zoltan. 
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