Serialized e-graph doesn't say which nodes are :unextractable

[oflatt]

As far as I can tell, the serialized e-graph from egglog doesn't tell you which constructors are unextractable. This is inconvenient for applications like eggcc with their own extractors. We could either mark things as unextractable in the metadata or we could have an option to omit these during serialization. I lean towards marking them so you can still have them in the same e-graph, eggcc uses unextractable stuff as analysis results during extraction.

Marking as bug because previously the e-graph did expose the function declarations so you could get this information.

[saulshanabrook]

We also currently do emit the cost. We could try adding like 'inf' or null or something there to mark a node as unextractable.