Function declarations in world.h moved to the bottom of the file.
Otherwise, there is a compile error, because struct World was referred to
in a function declaration before the declaration of struct World.
Refactored data structures used to represent the world, and entities
within the simulation. The overall entity type is now a union instead of a
struct, which is appropriate given that an entity is *either* a human,
infected or zombie, but only one at any given instant. The union fields
are the structs for representing those types directly (not pointers to
structs).

Also removed functions used to delete entities, as I plan to remove the
memory allocator. This allocation is effectively just a really dodgy and
bug ridden version of what malloc() and free() already do very well.