Refactor the EquipSet initialization into Ship::InitEquipSet
Remove redundant SlotSize definition
Export Ship::UpdateEquipStats to Lua
Use the correct property name for the mass calculations
Totally outsource the usedCargo and totalCargo handling to the EquipSet

Except for the initialization part, since the EquipSet cannot know of the
Ship as long as its constructor hasn't returned, because of the dynamic
type thingy for LuaObject.