Load Entities from RON

[ebkalderon]

We should be able to construct entities from YAML files (*.yml) stored in the resources/entities directory. Files can be nested arbitrarily in subfolders for organizational purposes, if the user wants. The markup format should be as follows:

---
component1:
    field1: true
    field2: 'value'
    field3: [1.0, 2.1]

component2:
    field1:
        - thing1
        - thing2
        - thing3

The string name of the entity is the filename minus the .yml extension. The user should be able to create an instance of such an entity in Rust code.

[msiglreith]

Remark: Relevant specs issue amethyst/specs#18

[ghost]

After taking a look at the ability to load a world/map in #145, I'm wondering how this will work for that use case. Using this approach for loading individual entities seems fine, but I think doesn't address the overall question of how to load a game world. It seems like we can proxy off most/all of the "real work" of loading entities/components to specs-rs, so if this looks right from Amethyst's POV, I can go propose this over there to see if there's any interest. Basically, it would look something like this:

impl State for SomeExample {
    fn on_start(&mut self, ctx: &mut Context, world: &mut World) {
        // other init code...

        let world_yaml = load_some_yaml_file("example");

        world.register_component(Component1, "component1");
        world.register_component(Component2, "component2");
        
        world.from_yaml(world_yaml["entities"]);
    }
}

With a resources/worlds/example.yml file that looks like this:

---
textures:
  # ...
entities:
  - component1:
      field1: true
      field2: 'value'
      field3: [1.0, 2.1]

  - component1:
      field1: false
      field2: 'value'
      field3: [1.0, 2.1]
    component2:
      field1: []

Also, I can move this to a new ticket if it seems off-topic from loading entities from yaml specifically.

[Xaeroxe]

Same as #122 (comment)

[Xaeroxe]

Ron is ready to use at this point and we've decided in other threads that's what we're going to be using, so I changed the issue title.

[zakarumych]

@Xaeroxe We should abstract away from any particular data formats where possible (and doesn't hit performance). Loading entities from any serde::Deserializer is better than just .ron
And I'd recommend using binary formats in production.

[Xaeroxe]

I'm downgrading this to "Important" because while valuable, this isn't something we can take immediate action on.

[torkleyy]

I don't see how this is much different from #122, but I think it can be closed, too.