Rewrite difficulty system

Fixes #80
I was pretty sure I wrote this comment, but can't find it anymore.

Instead of doing a manual for-loop, how about changing your table to provide the lookup naturally? You could use this:
```lua
local IsEntityBlockedInPeaceful =
{
  ["cZombie"] = true,
  ["cZombiePigman"] = true,
  ...
}

if (GetWorldDifficulty(World) == 0) then
  if (IsEntityBlockedInPeaceful[Entity:GetClass()]) then
    return true
  end
end
```