handle proof checking

[c-cube]

-[x] add (proof_produce " --some --additional --args $proof_file") to prover stanza

• add (proof_checker "checker $problem --whatever $proof") to prover stanza
  (or maybe a separate stanza with the (proof_checker <name>) field in prover)
• add a way in task/cli to specify that proof production is on for some provers; also need to be able to run the prover with and without proof production
• do the proof checking, with new possible results ("unsat but with bad proof", say)
• have a way to say "checked n subproofs, skipped m subproofs" (to say we have (n/n+m % steps proved)
• provide views on checked/unchecked proofs in the UI

[c-cube]

This is mostly done, although it'll need a lot of polish.