deserialize a ron file to an asset in AssetServer

[mockersf]

this PR makes it very easy to deserialise any struct as an asset from a ron file:

1. declare your asset

#[derive(Debug, Deserialize, TypeUuid)]
#[uuid = "abfa1180-abfa-41fa-80a1-2af49dd778fd"]
pub struct Character {
    name: String,
    hit_points: u32,
}

2. register it

app.add_asset::<Character>();

3. load it 🎉

let character: Handle<Character> = asset_server.deserialize("data/character.ron");

It depends on #1665 to work, and won't build without.

[alec-deason]

This would replace uglier custom code in my project, that's nice.

[huhlig]

This should likely be named something akin to load_ron to match the other load methods.

[mockersf]

closing with #1665