[m-rev.] for review: encode invariants in types when creating constant structures

[Zoltan Somogyi]

For review by anyone.

The main thing requiring review is the abort in the packing code
when it finds a double-word argument.

This cannot be triggered on 64 bit machines, which have no double word
primitive data types.

It cannot be triggered in hlc.pregen.gc, which boxes both floats
and int64/uint64 arguments.

I don't think it can be triggered for int64/uint64 fields on 32 bit machines,
since those are boxed there as well even when the grade is not pregen.

I think it can be triggered only on 32 bit machines in non-pregen grades
for float fields. However, I believe the existing code in ml_unify_gen.m
generated incorrect code for such fields already, because it treated them
as if their width was a single word. I haven't heard any reports of
such problems. Has anyone else?

Zoltan.
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