Destructive rewriting in egglog

[oflatt]

Destructive rewriting might be critical to making eggcc work when we are using unique ids. See thread in slack

Say we want to destructively rewrite loop A into loop B, which contains lots of stuff from loop A. Several problems in my mind
We’d like to re-use subexpressions without deep copying. I think this is okay, as long as we only narrow the set of possible inputs / memory states.
We need to “cleanly” replace loop A with loop B. How do we do this? Some sequence of union and delete?
We might leave behind garbage that isn’t referenced by other stuff (garbage collection needed?)