Added initial designs for Entity/Anim structs.

These are subject to change as I feel out the implementation, but
it's at least a good starting point.
Added useful `M_containerof()' macro.
Added another somewhat useful macro, `M_max()'
Implemented `iso_Entity', the first `iso_Drawable'

They are an attempt at capturing the most common use-case for
animated objects from a game-mechanics perspective.