fix SystemTime::elapsed()

[fusetim]

On wasm32 target, SystemTime::elapsed() was throwing error due to an inversion in the chronology of Instant.
duration_since need an earlier Instant in argument to work properly, and self should be before now (because we want elapsed time).
So the fix is:

    pub fn elapsed(&self) -> Result<Duration, ()> {
        SystemTime::now().duration_since(*self)
    }