Write an update_all_lockfiles script to go with "check_all_lockfiles" ?

It's too easy to forget to run "cargo update" in bench directories, and we too often forget to run it on the fuzzers.

We ought to have a script to run right after our version bumps, to update

As an alternative, we could consider not checking Cargo.lock files for our benchmarks into git. (Why do we do that at all? #1351 doesn't give the answers, since it's about fuzzing, not benchmarks. And we don't keep the fuzzers' Cargo.lock files checked in.)