Add serialization for inventories

[https123456789]

Add a set of methods to the inventory system API for serialization of a item registry.

Ideas

For serializing:

def serialize(self) -> String

And then to deserialize:

def deserialize(self, source: String) -> Inventory

This is planned to done (see this comment), but I wanted to make an issue to sort of remind me.

[LGgameLAB]

Me likey. I'm leaving this entirely up to you.

[https123456789]

Okay, I was planning on doing it anyway :sweat_smile:. Glad you like the idea though.