Refactor the egg-herbie interface a bit

[pavpanchekha]

In this PR make-egg-runner becomes make-egraph and run-egg becomes four functions, egraph-equal?, egraph-prove, egraph-best, and egraph-variations. This allows me to simplify preprocessing somewhat, analogous to the recent simplification of derivations.

I also leverage that fact that egg->herbie-dict and herbie->egg-dict are only used to rename variables, which means we don't need the dictionaries at all, just a normal Herbie context. One less thing to think about.