add a note about shrinkMutableByteArray
new PAP packing code

Old code was wasting a lot of space by packing tags for stack values
to indicate their pointer-hood. Instead, the new version packs the
original bitmap and uses it for unpacking.
The bitmap is actually present at the receiver side, but as part of
the function info table - the function arrives later so it cannot be
used from there.

There is code for the large bitmap case, but it is not activated yet.