new backend and save world

[Stephen Weeks]

While Matt was here, we discussed one problem with the new backend
using code pointers for return addresses instead of small integers.
What should be done with these addresses upon save/load world?  I
don't think that we can be guaranteed that these addresses preserve
their meaning across runs of the executable.  Any proposals?  Here are 
my two:

1. Use small integers in the native backend.
2. Rewrite stacks upon save world and load world.  When saving, turn
   the return addresses to small integers.  When loading, turn the
   small integers back to addresses.