Use new FastMoving class instead of Cavalry in AI/petra.
This class is an AI hint that such units are to be treated as moving fast, e.g. war dogs, or cavalry.
This makes it easier to introduce camels and chariots correctly.
Further work is required to make the AI unit choices less hardcoded.
Patch by: Nescio
Reviewed By: Angen, wraitii
Differential Revision: https://code.wildfiregames.com/D2251