[m-rev.] for review: check for type defn consistency when generating interface files

[Zoltan Somogyi]

For review by anyone.

There should be a new entry in notes/compiler_design.html
for the new module, but I am wondering whether the new module,
besides the new code now in it, should also contain the conversion
code between the various kinds of interface files, since that code
does not have much in common with the rest of prog_item.m.
If people agree, I would need a new name for the new module,
and would need to document that instead in compiler_design.html.
(Including that move in this diff would have made the diff much
harder to review.)

I would like to build on this change tomorrow, so in the absence
of both reviews and objections, I plan to commit it tomorrow
around lunchtime.

Zoltan.
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