Evolve status

I've just sent my current set of stable patches for the evolve command
to the list.

The code should be stable, but there's follow-up stuff I still want to do:
- Add more tests for the new code.
- Make better use of the memory pool in change-table.c (ideally, all
the memory it uses should come from the memory pool).
- Cosmetic non-functional touch-ups.

I may not have a chance to do more work on evolve for a while, and
since others have expressed an interest in helping with this work,
building on top of it, or just tinkering with metacommits - I thought
it would be good to make the code available to the community.