Add find_variants to Extractor

[bksaiki]

The EGraph type implements extract and extract_variants, but the extractor only implements find_best. Adds find_variants to Extractor which is basically a copy of the original extract_variants function.

[bksaiki]

As discussed with @oflatt and @Alex-Fischman. Unsure on the correct return type for Extractor::find_variants since find_best is an Option.

[codspeed-hq]

CodSpeed Performance Report

Merging #530 will not alter performance

_{Comparing bksaiki:extract-variants (23b5f90) with main (6f49428)}

Summary

✅ 10 untouched benchmarks

[Alex-Fischman]

I think that it's fine to not return a Result as long as you document that the *_variants functions might return fewer than limit values (if they don't exist in the e-graph).

[FTRobbin]

Looks good to me since it is mostly moving code from egraph to extractor.

But I don't understand the error message extract_variants should be called after extractor initialization. It seems to be an impossible reason unless there is some extractor initialization happening somewhere else.

[Alex-Fischman]

@bksaiki Can you confirm that you don't need this PR merged for the next month-ish? You're just upstreaming a change that herbie-egglog would like?

I would rather wait to merge this until the new backend is in; it seems just as easy to do this in the new backend as it is in this PR, and it will avoid merge conflicts.

[bksaiki]

Not high priority

[Alex-Fischman]

The new extractor in #571 will support this. It sounds like Herbie is not using this PR, so are we okay to close? OTOH if you do want it we can merge it before the new backend.

[Alex-Fischman]

Closing after confirming in-person.