Upgrade criterion

[dhardy]

• [x] Added a CHANGELOG.md entry

Summary

Built from #1329:

• Move rand_benches → benches and merge with rand_distr/benches.
• Use Cargo.lock.msrv to pin versions; remove direct pin on Rayon versions

[dhardy]

Note: we should probably eventually move all benchmarks to Criterion, but that's not a job for this PR.

To avoid name-clash, the non-Criterion distributions benchmark was renamed to base_distributions.