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

[Julien Fischer]

On Fri, 31 Jan 2020, Zoltan Somogyi wrote:

> 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.

That's fine.

Julien.