Breaks, labels, and trans for difftime

[paleolimbot]

There currently isn't any break, label, or trans functions for difftime objects. There are some excellent functions that do this for difftimes in the bench package that could be used for inspiration:

https://github.com/r-lib/bench/blob/3e5d63fbe263f3ced4b61715f21b951361bfef77/R/time.R

And some combination of these functions could be used to generate difftime breaks even for log scales (see r-lib/bench#50)