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/channel.py

@@ -94,9 +94,9 @@ class TextChannel(discord.abc.Messageable, discord.abc.GuildChannel, Hashable):
         :attr:`~Permissions.manage_messages` bypass slowmode.
     nsfw: :class:`bool`
         If the channel is marked as "not safe for work".
-        
+
         .. note::
-        
+
             To check if the channel or the guild of that channel are marked as NSFW, consider :meth:`is_nsfw` instead.
     """
 
@@ -894,9 +894,9 @@ class CategoryChannel(discord.abc.GuildChannel, Hashable):
         top category is position 0.
     nsfw: :class:`bool`
         If the channel is marked as "not safe for work".
-        
+
         .. note::
-        
+
             To check if the channel or the guild of that channel are marked as NSFW, consider :meth:`is_nsfw` instead.
     """
 
@@ -1096,9 +1096,9 @@ class StoreChannel(discord.abc.GuildChannel, Hashable):
         top channel is position 0.
     nsfw: :class:`bool`
         If the channel is marked as "not safe for work".
-        
+
         .. note::
-        
+
             To check if the channel or the guild of that channel are marked as NSFW, consider :meth:`is_nsfw` instead.
     """
     __slots__ = ('name', 'id', 'guild', '_state', 'nsfw',
@@ -1343,13 +1343,11 @@ class GroupChannel(discord.abc.Messageable, Hashable):
         The group channel ID.
     owner: :class:`User`
         The user that owns the group channel.
-    icon: Optional[:class:`str`]
-        The group channel's icon hash if provided.
     name: Optional[:class:`str`]
         The group channel's name if provided.
     """
 
-    __slots__ = ('id', 'recipients', 'owner', 'icon', 'name', 'me', '_state')
+    __slots__ = ('id', 'recipients', 'owner', '_icon', 'name', 'me', '_state')
 
     def __init__(self, *, me, state, data):
         self._state = state
@@ -1359,7 +1357,7 @@ class GroupChannel(discord.abc.Messageable, Hashable):
 
     def _update_group(self, data):
         owner_id = utils._get_as_snowflake(data, 'owner_id')
-        self.icon = data.get('icon')
+        self._icon = data.get('icon')
         self.name = data.get('name')
 
         try:
@@ -1393,40 +1391,11 @@ class GroupChannel(discord.abc.Messageable, Hashable):
         return ChannelType.group
 
     @property
-    def icon_url(self):
-        """:class:`Asset`: Returns the channel's icon asset if available.
-
-        This is equivalent to calling :meth:`icon_url_as` with
-        the default parameters ('webp' format and a size of 1024).
-        """
-        return self.icon_url_as()
-
-    def icon_url_as(self, *, format='webp', size=1024):
-        """Returns an :class:`Asset` for the icon the channel has.
-
-        The format must be one of 'webp', 'jpeg', 'jpg' or 'png'.
-        The size must be a power of 2 between 16 and 4096.
-
-        .. versionadded:: 2.0
-
-        Parameters
-        -----------
-        format: :class:`str`
-            The format to attempt to convert the icon to. Defaults to 'webp'.
-        size: :class:`int`
-            The size of the image to display.
-
-        Raises
-        ------
-        InvalidArgument
-            Bad image format passed to ``format`` or invalid ``size``.
-
-        Returns
-        --------
-        :class:`Asset`
-            The resulting CDN asset.
-        """
-        return Asset._from_icon(self._state, self, 'channel', format=format, size=size)
+    def icon(self):
+        """Optional[:class:`Asset`]: Returns the channel's icon asset if available."""
+        if self._icon is None:
+            return None
+        return Asset._from_icon(self._state, self.id, self._icon, path='channel')
 
     @property
     def created_at(self):