The way the trackers worked was rather... "adventurous", and while there
were some good ideas and it mostly worked, the implementation and
interface could do better.

Additionally, it was incomplete, for example there were a lot of
mechanics just missing.

While I'm not against having this functionality provided by evtclib, I
think it would be more worthwile with a better designed implementation &
API, so this "proof of concept" implementation is gone until there is a
better way of doing things.

gamedata is being kept, as the boss identifiers are useful and
applications shouldn't have to deal with keeping this low-level list
themselves.

thiserror seems to be the more modern approach that also works with the
new Error trait from std.

This was a change in Rust a while back, so we're updating to the new,
non-deprecated syntax.