Implement Mega Evolutions

[rhysm94]

Mega Evolutions are a specific kind of form change for Pokémon, where a Pokémon must be holding their Mega Stone (or know Dragon Ascent, in the case of Rayquaza), and the player must choose to activate it. Mega Evolution is usable once per battle. (For the avoidance of doubt, the process of Primal Reversion is functionally similar to Mega Evolution, but can likely be implemented as a separate form change condition)

This requires both #8 and #9 be fully implemented, for Pokémon to have Mega Evolutions as alternate forms, and to be able to hold Mega Stones.