There's a good chance that this will be evtclib 0.5, so we want to adapt
our API usage (mainly replacing evtclib::Boss with evtclib::Encounter).

The naming is a bit all over the place now, as we sometimes refer to
bosses and sometimes to encounters, but I hope to make a sensible
decision at *some point* about what we're actually doing here.

Since we have this information now anyway, might as well include it.
We're using humantime here, as that produces the expected "xxm yys zzms"
output. The conversion shouldn't fail (the unwrap), as we should never
encounter negative fight durations.

We changed the descending prefix to be ~ earlier, so now we need to
adjust the tests as well.