Add BeamExtract (approximate DAG extractor) and DagExpr (canonical multi-rooted DAG expression)

[recmo]

This PR adds two new structs:

• DagExpr. This generalizes RecExpr by allowing multiple roots. It also maintains strong invariants on minimality and canonical order, essentially being a unique representation of a given DAG for the roots.
• BeamExtract. A DAG extractor based on DagExpr that uses beam search to approximate the optimal.

One cool thing I realized while building this is that if we add a minor restriction on Language (the ordering of nodes is preserved under a monotonically increasing map of node Ids), then the merging two DagExprs can be done using a linear time merge sort variant.

I'm wondering if this minor restriction should be added to the docs of Language as a requirement, or an additional marker trait languages need to implement.

It may make sense to use DagExpr for the LpExtract result as well, but that would be breaking the public API.

[oflatt]

Hi- thanks for your PR!
The egg team has switched focus to egglog and can't keep expanding egg.

Could this PR be a separate crate?

This algorithm could be a great contribution to the extraction gym! It works over a serialized egraph that both egg and egglog export to.

https://github.com/egraphs-good/extraction-gym