Entity commands and more

[MarcGuiselin]

A large refactor adding support entity-commands and more. New WIT Features:

• The entity resource, which holds and obfuscates the id. This has no constructor. It guarantees mods can only access entities they have permission to.
• query.iter returns query-result, giving access to a getter for entity
• New useful aliases such as bundle for serialized component lists
• Implement command.spawn-empty
• Implement command.entity
• command.spawn returns entity-commands
• Implement entity-commands resource
  • id returns an entity
  • insert a bundle
  • remove a bundle
  • despawn
  • try-despawn
• The app constructor has been removed. Instead, the setup function receives an app as a parameter. Now Mod authors have no way to construct and access Mod outside the setup function.

The hosts directory in particular received big refactors to make code clearer, break down monolithic functions and enable more code reuse.

Depends on #33