use monotonic clock instead of system clock

[bergey]

closes #13

[bergey]

After benchmarking this change, I'm not sure I can recommend it. I'm seeing 2.5 times as much overhead from this version as from the system clock version in master. On my recent Macbook Pro, I see 400us median overhead on master, 1000us on this branch, and 800us if I use the monotonic clock but convert directly to Double without Rational . Those seem like overheads that might concern some users of this library. What do you think?

[domenkozar]

How about using https://hackage.haskell.org/package/base-4.12.0.0/docs/GHC-Clock.html#v:getMonotonicTimeNSec that should be significantly faster?

[bergey]

@domenkozar Indeed, GHC.Clock works much better. Thanks! Closing in favor of #25