Serialize the player's inventory in save files

[patowen]

This completes part of #428.

Allows users to exit and re-open Hypermine while keeping the contents of their inventory, as previously, the inventory was cleared every time the game was loaded.

[Ralith]

Are child entities not also in graph_entities?

[patowen]

Not currently, although given how tricky this implementation is, I would consider this to be a mistake on my part.

There's a distinction between entities that have an inherent position (which is managed with a Position component) and nodes that don't, such as inventory items. For the latter, they belong to an entity that does have a position.

I had decided that graph_entities would only contain entities that have a position, but I have a good reason to second-guess that, since that doesn't correspond with how save files are organized at all. I'll go ahead and fix this before my next push to this branch.

EDIT: Actually, I might have to think about this some more.

[patowen]

I've updated the PR so that child entities are now in graph_entities. To accomplish this (what I believe to be) the most understandable way possible, I've made NodeId into its own component intrinsically linked with graph_entities so that inventory items can be associated with a NodeId without needing a Position.

This is helpful because Position has a bunch of other functionality, such as syncing with clients, so separating this functionality is important.

I'll resolve this thread for now, as child entities now are in graph_entities, but the resolution may be controversial, and I'm definitely open to feedback.