This causes issues with the player: prefix used in any() and all()
constructs, as player: will now be parsed as a word instead of the
proper token. For now, : is disallowed in words again until there is a
better solution.

Away with stringly typed stuff, we now have a proper way to save the
profession of a player without relying on a string. Theoretically, that
is better for memory consumption, as we now save only the identifier and
use fmt::Display and static strings, but that was not the main reason
for this change.

The main reason is that now we can programatically work with the
profession and elite spec, so that we can (for example) implement a
filter to filter players based on their class.

The word "class" has been chosen because it is a common synonym for the
profession/elite, and because this is neither a profession nor the elite
- it's a combination of both.

Rust doesn't necessarily need this, but it's good formatting to include
it anyway.

With the file name heuristic for -before and -after in place, we might
want a way for the user to disable it. For now, we simply do this by
providing a new set of predicates without the filter.

In the future, we might have a --disable-heuristics switch to disable
the heuristics, in case we ever add more.