[m-rev.] for review: pragma consider_used

[Zoltan Somogyi]

The only things that needs review are the concept and the name;
the implementation is straightforward.

I will add public documentation once we have a consensus on
the name.

As the log message says, I have tested the diff on stratify.m.
After two or three weeks, when this diff has been installed
on all our development systems, I intend to add these pragmas
to the compiler itself, so separate test cases aren't needed.

Zoltan.
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