Moved to `time.h`.

Currently, `Time` can be either an absolute time (when returned by `steadyTime()` and as a parameter to `atSteadyTime()`) or a duration (when passed to `atTimeFromNow()`). The correct solution would be to have two types for this, Time and Duration, in the same way as in `std::chrono`. I'm not sure how well `Quantity` supports this use case.