Integrate assets into the saving/loading process

[tlein]

Currently to load an asset we use a pattern where we add _key member to the object. The key is the used in the FetchDependencies phase to load the asset. If we integrated the Assets into the loading framework we could get rid of the _key value.

Ex of what the API would look like:

sf::Texture _texture;
arc.asset(_texture, "asset-key");