-gamemode is a more general version of -raid/-fractal/...

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 allows things like "2m 50s" but also "5m".

humantime would allow even more, but we need a regular expression to
catch them for lalrpop, so for now this are the only two supported
formats (together with "50s"). Considering the usual time of fights in
GW2, I doubt we'll need anything bigger than minutes.

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.

With the addition of our own parser, we no longer need the FromStr
implementation for SearchField. Furthermore, it is now only used in
player::NameFilter, so the definition has been moved there. The import
in the grammar was unused as well, so it has been removed.