Add an atime CI job and delete stale benchmarking vignettes

[MichaelChirico]

The files under data/, e.g. https://github.com/r-lib/bit64/blob/cb4552b8345550d158cc7ae9b937b009f4dcd200/data/benchmark64.data.rda, are stale.

Rather than continually updating (and shipping) these .rda files, better IMO to track performance as part of CI.

For this I am most familiar with the {atime} setup that {data.table} is using.

See also the timings being removed as part of #43