Entities should be roughly split into a body, an inventory and an AI.
The AI determines the goals of the entity.
The body determines how targets can be reached (swimming, walking or flying) and some general stats of the entity.
The inventory determines what tools and attacks the entity has.

All combinations of these should be allowed and additionally the player should be allowed to take control of different bodies.

Sub-tasks:

• Collision Detection Collision Detection #82
• Friction Friction and Air resistance #83
• Buoyancy Buoyancy #84
• Swimming Swimming #85
• Entity AI Entity AI #86
• Animation Entity Animation #87
• Entity Struct or ECS?
• Per-chunk entity storage
• Entity saving/loading
• Models