Inheritance and wrapped mtype

As an example of how the mtype wrapper can be used, consider this `main.lua`:

```lua
monster_types = { 
    "mon_zombie", 
    "mon_zombie_fat", 
    "mon_zombie_tough", 
    "mon_zombie_crawler", 
    "mon_zombie_hazmat", 
    "mon_zombie_fireman", 
    "mon_zombie_survivor", 
    "mon_zombie_rot", 
    "mon_zombie_cop", 
    "mon_zombie_shrieker", 
    "mon_zombie_spitter", 
    "mon_zombie_electric", 
    "mon_zombie_smoker", 
    "mon_zombie_swimmer", 
    "mon_zombie_dog", 
    "mon_zombie_brute", 
    "mon_zombie_hulk"
}

-- halve the speed of all monsters in the above list
for _, monster_type in ipairs(monster_types) do
    game.monster_type(monster_type).speed = game.monster_type(monster_type).speed / 2
end
```

With this, one can create a mod that will e.g. halve the speeds of all zombies.