Ideas for a new format for heap images.

Number the exported arenas from 0..n (<= 255).
Whe writing a pointer p out, adjust it as follows:

    ((ArenaId << 24) | (p - arenaBase))

Adjusting the pointers on input is then simply a
matter of looking up the new base address in
a table and adding it to the low 24 bits.

If an arena is bigger than 2^24 (16Mb), then use
multiple ArenaIds.  Since the current number of
arenas is limited to be at most 70, this scheme
can support a minimum of 2976Mb.  Since this
exceeds the capacity of most disks, we should be
okay.

The tricky part of this is that when exporting, the
bigobject references must be mapped into a coalesced
big-object region.  We can handle this by allocating
an offset word per big-object descriptor.

