show operation time / duration in nanoseconds

[milahu]

currently, only the operation frequency is displayed in operations per second = hertz

to me, getting the operation time feels more natural
like code A needs 10 nanoseconds, code B needs 100 nanoseconds
(smaller is better)

its just more intuitive than comparing
100.000 ops/sec vs 10.000 ops/sec
(larger is better)

also, when you time different components of one larger program
you can easily summarize the time-values

ideally make this optional, so we can switch between time and frequency
maybe with a per-test preset config, stored in the github-gist