Rewrite Asset design

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.

discord/flags.py

@@ -504,7 +504,7 @@ class Intents(BaseFlags):
         - :attr:`Member.nick`
         - :attr:`Member.premium_since`
         - :attr:`User.name`
-        - :attr:`User.avatar` (:attr:`User.avatar_url` and :meth:`User.avatar_url_as`)
+        - :attr:`User.avatar`
         - :attr:`User.discriminator`
 
         For more information go to the :ref:`member intent documentation <need_members_intent>`.