Add a `rationalToElapsedP` function

[ad-si]

For example:

rationalToElapsedP :: Rational -> ElapsedP
rationalToElapsedP secondsFrac =
  let (sec, nanoSec) = properFraction secondsFrac
  in ElapsedP (Elapsed (Seconds sec)) (NanoSeconds $ truncate $ nanoSec * 1e9)