Close #168: Move CharacterStats out of Battler, to PartyMember

The code has grown pretty complex. Diving back into the battler, rewards, etc.,
I found them hard to work with and some bits weren't logical. E.g. using the
porcupine's experience stat (earned experience) as the player's experience
reward upon killing them!

I missed some of these during code reviews. Anyway that's part of the coding
game: this demo is preproduction work, we have to try ideas and refactor as we
gain experience.

This breaks the rewards at the moment, as they were coupled with the battlers'
stats. It also breaks monsters. I'm opening issues to reflect that.