Hashmap visit asset dependencies impl

[RylanYancey]

Objective

• Allow tagging of HashMaps with #[dependency], for assets which contain variable binding assignments to asset handles.

Solution

• The solution is to create an implementation of VisitAssetDependencies for HashMap that behaves similarly to HashSet.

Testing

• The changes were tested by adding map_handles and untyped_map_handles to StructTestAsset and EnumTestAsset. Then, cargo test was ran in crates/bevy_asset.

Showcase

I plan to use this to create a map of material bindings to material handles. In the below example, materials is a map of bindings (e.g. "$east", "$south"). Each Element contains up to six references, so this helps when there are block templates that only use one material for all faces.

#[derive(Asset, Reflect, Clone)]
#[type_path = "ovx::blocks::template"]
#[type_name = "BlockTemplate"]
pub struct BlockTemplate {
    #[dependency]
    pub inherits: Vec<Handle<BlockTemplate>>,
    #[dependency]
    pub materials: HashMap<String, Handle<BlockMaterial>>,
    pub elements: HashMap<String, Element>,
    pub variants: HashMap<String, Variant>,
    pub states: HashMap<String, BlockState>,
}

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