In-battle stats and stage multipliers

[kipyin]

Issue #15 suggested the implementation of in-battle stats, but none of the then-existing codes actually used it, hence there is a few things that I was missing when I wrote BattleStats:

1. The stage multipliers needs to be tracked, because some moves (e.g. Punishment) need to use this info.

2. The stage multipliers actually also include the critical-hit rate, which was not included when implementing BattleStats.

Hence, to implement above mechanisms, we have two choices:

1. Make a new class, StatStages, and put the stage info into this new class.

2. Make a _Stat class, which represent a single stat, such as hp or attack, which contains the value of its permanent stats, effective stats, stage, and stage change history. Remove BattleStats class. Access all stat-related data like Pokemon().stats.hp.permanent, Pokemon().stats.evasion.effective, Pokemon().stats.critical_hit.stage_log, etc. Of course, Pokemon().stats.critical_hit.permanent will be a non-sense, and will probably be 0.

[kipyin]

If we take the first choice, we would have at least 3 stat-related properties in Pokemon: Pokemon.stats, Pokemon.battle_stats, and Pokemon.stages (and even Pokemon.stage_logs).

If we take the second choice, then we will end up with deeply stored properties.

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We can also set BattleStats.attack etc. as a named tuple: BattleStats.attack = Stat(value=100, stage=-2)