Dependabot warnings for crossbeam-utils and regex

[carlosbalbora]

I am using criterion.rs 0.3.6 that it uses crossbeam v0.8.6 (via rayon 1.5.1) and regex regex v1.5.4. It throws high level warnings:

• error for crossbeam Fix unsoundness of AtomicCell<*64> arithmetics on 32-bit targets that support Atomic*64 crossbeam-rs/crossbeam#781
• error for regex https://groups.google.com/g/rustlang-security-announcements/c/NcNNL1Jq7Yw
  But bugs can be fixed if we update rayon and regex (I can do the PR myself)

[carlosbalbora]

#604 <- my PR

[bheisler]

Thanks for the info and the pull request, but my policy is to not accept pull requests that only update dependency versions.

I considered making an exception but it seems that Cargo already happily pulls in and uses the new versions of those two dependencies even with the Criterion-rs specifying a dependency on the old version, so there doesn't seem to be any particular urgency as users can simply update their lockfiles and get a newer version of regex and rayon.