Add mtype::hp to lua class bindings.

Fixes http://smf.cataclysmdda.com/index.php?topic=8750 so MorCel 374 can implement their mod.

Tested with this lua script (copy of the slow zombie mod):
```
for _, monster_type in ipairs(monster_types) do
    if monster_type:find("mon_zombie") then
        game.monster_type(monster_type).hp = 1
    end
end
```