Add a replace action

[oflatt]

When writing rules where one row in a table subsumes another, it's often hard to write the rules. This PR adds a prototype for a replace action to solve this problem.

In particular, the only way to do it right now is to use delete. However, if the two rows are actually the same, the problem is that we never saturate.
Stupid example that runs forever:
https://egraphs-good.github.io/egglog/?program=XQAAgACeAAAAAAAAAAAFYJzIYToE-74urWhpV-IA_[…]IY2FIeFaKKvnanDvE9ubGR2gphrxqDRu1WAeyBGxkXMQP_-wXIAA%253D

This PR fixes this problem by adding a replace action, allowing for one row to replace another (and does nothing when they are the same).

[oflatt]

Maybe we should get rid of delete in favor of replace?

[yihozhang]

Since replace desugars to a set in actions, do you need to also duplicate it in query to make semi-naive sound?

[oflatt]

Since replace desugars to a set in actions, do you need to also duplicate it in query to make semi-naive sound?

Yes, great catch! Fixed.

[oflatt]

To be clear, we shouldn't merge this yet- still playing around with it.

[mwillsey]

Let’s keep not-ready prs as drafts, it helps reduce notification noise a bit

[oflatt]

No longer necessary for terms- could still be a good idea but closing for now.