[m-rev.] for review: reorder the contents of map.m

[Zoltan Somogyi]

This puts the functions and predicates in map.m into groups of related
code, and puts the groups into order roughly from simpler to more complex
functionality. This is similar to what I did some months ago for the set modules.
There are no changes except reordering, adding comments to a few predicates
that did not have them, and a few fixes to the existing comments, so posting
a diff would be useless. I am posting the whole new file instead, but only the
interface needs review, by anyone.

I am doing this because I want to add a new predicate, map.keys_as_set
(which will be a separate change), and found that map.keys was surrounded
by unrelated predicates.

Zoltan.
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