[m-rev.] for review: pack sub-word-sized integers and dummies

[Zoltan Somogyi]

For review by anyone.

I have bootchecked a pre-final-cleanup version of this diff
3x2 times: in asm_fast.gc, asm_fast.debug.gc.stseg, and hlc.gc,
and in both 64 and 32 bit mode. The only failures were two tests
in debug grades that fail even without this diff.

I will repeat all six bootchecks before final commit.

When it comes to testing this diff, please remember I committed
an extensive test case intended for this about a week ago.

Zoltan.
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