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.

It makes sense to unify this implementation to avoid code duplication
and bugs that might be hidden. -after and -before can stay for now, as
shortcuts for -time < and -time >, the same way we have other shortcuts
as well.

This enables filters such as
    -time > 2020-01-01
    -time < 2020-02-03
    ...
for time and duration, and later possibly also for more things (such as
a COUNT(...) construct).

This work tries to integrate them into the existing filter system as
seamless as possible, by providing a Comparator which implements
LogFilter.

The "type checking" is done at parse time, so nonsensical comparisons
like -time > 12s flat out give a parse error. This however might be
changed to a more dynamic system with run-time type checking, in which
case we could do away with the type parameter on Producer and simply
work with a generic Value. The comparator would then return an error if
two non-identical types would be compared.

Note that the system does not support arithmetic expressions, only
simple comparisons to constant values.