This is a very rough version of the inventory module and associated features. This will be a good base to start building something more robust.
- adds a player inventory to all clients
- makes clients keep track of what inventory they have open
- adds very naive state sync for inventories
related: #53
# Test plans
### Building
1. run `building` example
2. pick any block from creative
3. place it
4. pick a different block from creative
5. place 2
6. break the last one
7. see that both remaining blocks don't become dirt
### Chest inventories
1. run `chest` example
2. sneak to give yourself a stone block
3. open the chest (it's invisible on top of the stone block)
4. place stone in chest
5. see the stone moving around in the chest
## Known Issues
To be fixed in later PRs
- It's very easy to duplicate items if you take the stone out of the chest on the same tick as the server trying to move the item.
- The chest is invisible when you spawn in. It appears if you break it.
- `get_slot` and `set_slot` can panic.
