This is a breaking change.
This does the following transformations, assuming `asset` represents
an asset type.
Object.is_asset_animated() => Object.asset.is_animated()
Object.asset => Object.asset.key
Object.asset_url => Object.asset_url
Object.asset_url_as => Object.asset.replace(...)
Since the asset type now requires a key (or hash, if you will),
Emoji had to be flattened similar to how Attachment was done since
these assets are keyed solely ID.
Emoji.url (Asset) => Emoji.url (str)
Emoji.url_as => removed
Emoji.url.read => Emoji.read
Emoji.url.save => Emoji.save
This transformation was also done to PartialEmoji.
