Load character entities from the save file

[patowen]

This PR sets up the initial implementation of loading entity data from a save file by loading player characters. In single-player, this has the effect of having the game remember where you were when you close it. If you change your name, you will spawn in as a new character.

The following are not currently implemented:

• Saving client-authoritative orientation data (the orientation quaternion)
• Only loading entities in nearby chunks (Currently, everything is loaded.)
• Saving the inventory